Measles is associated with suppression of mitogen-induced proliferative responses. The role of monocytes in depressed responses to phytohemagglutinin during measles was assessed. Depleting peripheral blood mononuclear cells of adherent cells decreased, increased, and did not change proliferative responses in essentially equal numbers of individuals. Inhibition of prostaglandin synthesis by indomethacin significantly increased proliferation (P = 0.009) but often not to normal values. Addition of supplemental interleukin 1 had little effect. Addition of supplemental natural interleukin 2 significantly increased proliferation (P = 0.002) even in patients with very low responses. These data suggest that monocyte function is abnormal in some individuals with measles, that the abnormality is variable, and that lymphocyte response to monocyte signals is probably suboptimal as well. © 1987.