PD-1 expression by tumor-associated macrophages inhibits phagocytosis and tumor immunity. This study shows that both mouse and human tumor-associated macrophages (TAMs) express PD-1, and PD-1 expression increases with disease progression. TAMs with high PD-1 expression have reduced phagocytic activity, and blocking PD-1/PD-L1 in vivo increases macrophage phagocytosis, reduces tumor growth, and extends survival in mouse models. These findings suggest that PD-1/PD-L1 therapies may also function through a direct effect on macrophages, with significant implications for treatment with these agents. The presence of TAMs correlates with poor prognosis in human cancers. Recent work has shown that macrophages can be induced to phagocytose tumor cells through SIRPα/CD47 blockade, and this therapeutic strategy is currently the subject of multiple clinical trials in cancer. Although SIRPα/CD47 may serve as a primary regulatory “checkpoint” on macrophages, other immune-regulatory receptors could serve a complementary or redundant role. The PD-1 receptor is one of the best-studied and most clinically successful immune checkpoint drug targets, but its primary function is widely understood to be in the regulation of T cells. However, given that macrophages have previously been reported to express PD-1 in the context of pathogen infection, we wondered whether macrophages might also express PD-1 in the tumor microenvironment, and if so, what consequences this expression might have on anti-tumor immunity. To assess PD-1 expression on TAMs in an immunocompetent syngeneic setting, we used the colon cancer line CT26. Fluorescence-activated cell sorting (FACS) of dissociated tumors 3 weeks post-engraftment showed that indeed, a high percentage of macrophages in the tumor expressed surface PD-1. Immunofluorescence (IF) revealed a clear and abundant population of cells expressing both the macrophage marker CD68 and PD-1. Flow cytometry analysis revealed that virtually all PD-1⁺ TAMs expressed an M2-like surface profile. Further analysis of mouse CT26 tumors in syngeneic hosts revealed that this PD-1⁺ TAM population is not static; it begins to emerge circa 2 weeks post-engraftment, and increases over time. We found that PD-1 expression correlated strongly with time post-engraftment and with tumor volume. Given these observations in mice, we wondered whether human macrophages similarly express PD-1 in the primary tumor setting. Upon profiling the TAMs in human colorectal cancer samples, we saw high but variable PD-1 expression on human TAMs. Strikingly, we also observed that, just as in mice, the M2 population expressed significantly more PD-1 than the M1 population.PD-1 expression by tumor-associated macrophages inhibits phagocytosis and tumor immunity. This study shows that both mouse and human tumor-associated macrophages (TAMs) express PD-1, and PD-1 expression increases with disease progression. TAMs with high PD-1 expression have reduced phagocytic activity, and blocking PD-1/PD-L1 in vivo increases macrophage phagocytosis, reduces tumor growth, and extends survival in mouse models. These findings suggest that PD-1/PD-L1 therapies may also function through a direct effect on macrophages, with significant implications for treatment with these agents. The presence of TAMs correlates with poor prognosis in human cancers. Recent work has shown that macrophages can be induced to phagocytose tumor cells through SIRPα/CD47 blockade, and this therapeutic strategy is currently the subject of multiple clinical trials in cancer. Although SIRPα/CD47 may serve as a primary regulatory “checkpoint” on macrophages, other immune-regulatory receptors could serve a complementary or redundant role. The PD-1 receptor is one of the best-studied and most clinically successful immune checkpoint drug targets, but its primary function is widely understood to be in the regulation of T cells. However, given that macrophages have previously been reported to express PD-1 in the context of pathogen infection, we wondered whether macrophages might also express PD-1 in the tumor microenvironment, and if so, what consequences this expression might have on anti-tumor immunity. To assess PD-1 expression on TAMs in an immunocompetent syngeneic setting, we used the colon cancer line CT26. Fluorescence-activated cell sorting (FACS) of dissociated tumors 3 weeks post-engraftment showed that indeed, a high percentage of macrophages in the tumor expressed surface PD-1. Immunofluorescence (IF) revealed a clear and abundant population of cells expressing both the macrophage marker CD68 and PD-1. Flow cytometry analysis revealed that virtually all PD-1⁺ TAMs expressed an M2-like surface profile. Further analysis of mouse CT26 tumors in syngeneic hosts revealed that this PD-1⁺ TAM population is not static; it begins to emerge circa 2 weeks post-engraftment, and increases over time. We found that PD-1 expression correlated strongly with time post-engraftment and with tumor volume. Given these observations in mice, we wondered whether human macrophages similarly express PD-1 in the primary tumor setting. Upon profiling the TAMs in human colorectal cancer samples, we saw high but variable PD-1 expression on human TAMs. Strikingly, we also observed that, just as in mice, the M2 population expressed significantly more PD-1 than the M1 population.