The effect of human transfer factor (TF) or its components L-serine and/or glycine in tuberculin (PPD), or leucoagglutinin (LA) induced leucocyte migration inhibitory factor (LIF) secretion was studied. Augmentation of LIF secretion was seen with low concentration ( = 0.078 g/l) of TF when lymphocytes were cultured in minimum essential medium for suspension cultures (MEM-S), a culture medium lacking L-serine and glycine. High concentrations (0.3125-5.0 g/l dry weight) of TF were inhibitory in MEM-S. In RPMI 1640, a culture medium containing L-serine and glycine, TF was either inhibitory or had no effect. The combination of L-serine and glycine, at concentrations equivalent or lower than the optimum of TF, had an augmenting effect on LIF secretion identical to that of TF, but no inhibition at higher concentrations was seen. The results indicate that human TF contains components which have suppressive or augmenting effects on LIF secretion in vitro. The augmenting effect may be mainly due to L-serine and glycine and thus not related to TF's activity in vivo.