The thiol-alkylating agent, N-ethylmaleimide (NEM), was found to inhibit the response of human peripheral blood mononuclear cells to the T-cell mitogen, concanavalin A (Con A). NEM (10 μM) blocked Con A-induced agglutination, production of interleukin-2 (IL-2) and expression of the IL-2 receptors (Tac) without toxicity. In order to determine whether the effectsof NEM on lymphokine production were related to inhibition of agglutination, we compared the immunosuppressive effects of NEM with those of cytochalasin A, cytochalasin B and colchicine. NEM did not inhibit E-rosette (ER) formation, suggesting that it does not interfere with actin filaments. Low concentrations of NEM (4 μM) inhibited IL-2 production and Tac expression without inhibiting agglutination, while 6–10 μM NEM blocked agglutination and DNA synthesis as well. In contrast, 5–10 μM cytochalasin B (CB) inhibited ER formation, agglutination, Tac expression and DNA synthesis, but augmented IL-2 production by three- to ten-fold. Colchicine (0.1–10 μM) had no effect on ER formation or agglutination and augmented IL-2 production by as much as 18-fold. However, colchicine blocked Tac expression by >40% and DNA synthesis by >80%. Cytochalasin A (CA), which has the thiol-reactive properties of NEM, the actin filament-disrupting properties of CB, and the microtubule-distrupting properties of colchicine, exhibited the immunosuppressive effects of all three compounds. These studies suggest that the inhibitory effects of NEM on IL-2 production do not appear to be due to reactivity with the cytoskeleton, but are probably due to effects on signal transduction pathways leading to IL-2 production and expression of IL-2 receptors.