This paper investigates the role of macrophages in potentiating the T-lymphocyte response to mitogens. The authors demonstrate that macrophages release soluble factors, which they term "lymphocyte-activating factors" (LAF), that enhance the mitotic response of mouse thymocytes to phytohemagglutinin (PHA). These factors are not mitogenic for B cells and are produced in response to agents like endotoxin. The study also shows that LAF acts on both central and peripheral T cells, potentiating their responses to PHA and concanavalin A (Con A). The findings suggest that LAF may act as a mitogen or a nutritional factor, affecting the metabolic state or receptor activity of the target cells. The results extend previous work showing the involvement of similar factors in specific immune responses and provide a mechanism for the role of macrophages in lymphoid cell interactions.This paper investigates the role of macrophages in potentiating the T-lymphocyte response to mitogens. The authors demonstrate that macrophages release soluble factors, which they term "lymphocyte-activating factors" (LAF), that enhance the mitotic response of mouse thymocytes to phytohemagglutinin (PHA). These factors are not mitogenic for B cells and are produced in response to agents like endotoxin. The study also shows that LAF acts on both central and peripheral T cells, potentiating their responses to PHA and concanavalin A (Con A). The findings suggest that LAF may act as a mitogen or a nutritional factor, affecting the metabolic state or receptor activity of the target cells. The results extend previous work showing the involvement of similar factors in specific immune responses and provide a mechanism for the role of macrophages in lymphoid cell interactions.