Macrophages play a crucial role in antigen recognition by guinea pig T lymphocytes. The study shows that T lymphocyte proliferation in response to antigen requires the presentation of antigen on histocompatible macrophages. This interaction can be blocked by alloantisera, indicating that histocompatibility determinants or membrane surface products linked to the major histocompatibility region are involved. The study used inbred strain 2 and 13 guinea pigs and demonstrated that syngeneic macrophages are necessary for efficient antigen presentation to T lymphocytes. Alloantisera directed against histocompatibility antigens on both macrophages and T lymphocytes inhibit the proliferative response, while those targeting antigens on only one cell type do not. The findings suggest that macrophage-lymphocyte interaction is regulated by histocompatibility antigens or products of genes closely linked to the major histocompatibility complex. The study also shows that outbred guinea pigs can activate T lymphocytes if they share histocompatibility determinants with the lymphocytes. The results indicate that macrophages are essential for antigen presentation to T lymphocytes, and their interaction is mediated by histocompatibility antigens. The study highlights the importance of histocompatibility in macrophage-lymphocyte interactions and the role of macrophages in antigen recognition by T lymphocytes.Macrophages play a crucial role in antigen recognition by guinea pig T lymphocytes. The study shows that T lymphocyte proliferation in response to antigen requires the presentation of antigen on histocompatible macrophages. This interaction can be blocked by alloantisera, indicating that histocompatibility determinants or membrane surface products linked to the major histocompatibility region are involved. The study used inbred strain 2 and 13 guinea pigs and demonstrated that syngeneic macrophages are necessary for efficient antigen presentation to T lymphocytes. Alloantisera directed against histocompatibility antigens on both macrophages and T lymphocytes inhibit the proliferative response, while those targeting antigens on only one cell type do not. The findings suggest that macrophage-lymphocyte interaction is regulated by histocompatibility antigens or products of genes closely linked to the major histocompatibility complex. The study also shows that outbred guinea pigs can activate T lymphocytes if they share histocompatibility determinants with the lymphocytes. The results indicate that macrophages are essential for antigen presentation to T lymphocytes, and their interaction is mediated by histocompatibility antigens. The study highlights the importance of histocompatibility in macrophage-lymphocyte interactions and the role of macrophages in antigen recognition by T lymphocytes.