Pretreatment with D-Penicillamine of rat peritoneal macrophages markedly influences their ability to modulate Concanavalin A stimulated [3H]thymidine incorporation in non-adherent rat lymph node cells. Untreated macrophages added in high numbers to the lymphocytes (ratio lymphocytes/macrophage 7.5 : 1) depress their [3H]thymidine incorporation. Pretreatment with D-Penicillamine further reduces this response. Lower proportions of untreated macrophages (ratio 15 : 1) exert a stimulatory effect on the lymphocytes. This response is reversed by pretreatment of macrophages with D-Penicillamine. Addition of untreated macrophages in very low proportions (40 : 1 and 75 : 1) is without effect on the [3H]thymidine incorporation by the lymphocytes. This response is markedly stimulated by pretreatment of the macrophages with D-Penicillamine. The culture of macrophages in the presence of D-Penicillamine increases their incorporation of [3H]D-glucosamine, but it is without effect on their incorporation of [3H]L-leucine. It is suggested that D-Penicillamine enhances the functional activity of the macrophage in the regulation of the lymphocyte response to mitogens.