Free essential fatty acids (EFA) are reported to suppress cell-mediated immunity. Because Intralipid contains a high concentration of esterified EFA, the effects of this emulsion on in vitro lymphocyte transformation were studied. Intralipid concentrations of 11.5, 115, and 230 mg% in lymphocyte cultures increased phytohemagglutinin (PHA) stimulation by an average of 8.2% (not significant [NS]), 18.1% (p < 0.01), and 11.8% (NS), respectively. These same concentrations also increased Varidase stimulation in lymphocyte cultures by an average of 11.3 (p < 0.02), 18.9 (p < 0.02), and 4.4% (NS), respectively. Control wells did not demonstrate allergic reactions to Intralipid. These data demonstrate that Intralipid can significantly increase the mitogenic response of human thymic lymphocytes and the antigenic response of human lymphocytes, in vitro.