Murine amniotic fluid (MAF), alpha-foeto-protein (AFP) and MAF depleted of AFP by affinity chromatography (MAF-AFP) inhibited the T-cell dependent in vitro proliferative responses of lymph node cells sensitized to a variety of soluble antigens. Variable degrees of inhibition were observed with the different antigens used in the assay. In general, the higher the proliferative response induced by a particular antigen, the less it was inhibited by the three inhibitors. Enhancement of proliferation was not infrequently observed at lower concentrations followed by a dose-dependent inhibition as the concentration of the inhibitor was increased. Usually the order of inhibition was MAF greater than MAF-AFP greater than or equal to AFP although variations in inhibitory potency were noted between different preparations of AFP and MAF-AFP. The existence of inhibitors in preparations of MAF depleted of AFP raised the question as to whether MAF contains single or multiple inhibitory factors. The most facile explanation is that two inhbitors exist; AFP and the as yet uncharacterized non-AFP suppressor present in MAF-AFP.