Free fatty acids (FFA) were tested for their effects on in vitro thymus-dependent (TD) and thymus-independent (TI) immune responses. Murine T cells proliferating in one-way mixed leucocyte reactions (MLR) were extremely sensitive to inhibition by exogenous stearic acid (18:0) but were only moderately affected by oleic acid (18:1). T-cell proliferation was suppressed when 18:0 was added as late as 44 hr after allogeneic stimulation, but sensitivity to 18:1 was limited to the first 30 hr of culture. The inhibitory effects of 18:0, but not 18:1 were potentiated by concomitant T-cell activation and under such conditions the effects of 18:0 were irreversible within 5 hr. The two fatty acids were additionally tested for their effects on the anti-hapten antibody-secreting cell responses to TD and TI antigens. Both 18:0 and 18:1 inhibited the primary antibody response elicited by a TD antigen (TNP-KLH) but neither fatty acid significantly affected the primary antibody response to a TI antigen (TNP-LPS). Following in vivo immunization with TNP-KLH, isolated spleen cells were challenged with the same antigen in vitro in the presence of FFA. Whereas 18:1 had little effect on the secondary immune response, the addition of 18:0 led to a 3-4-fold increase in the number of anti-TNP plaque forming cells. Further studies showed that TNP-KLH-induced T cell proliferation was potently inhibited by 18:0 but 18:1 had no effect. These results suggest that an inhibition of T-cell proliferation is the primary way in which 18:0 modulates TD immune responses in vitro. By contrast, 18:1 appears to inhibit primary antibody responses and MLR via alternative mechanisms.