Cord blood lymphocytes, generally believed to be immunologically uncommitted, were used for investigating the possible conversion by transfer factor (TF) of non-immune to immune lymphocytes in vitro. TF was prepared from pooled buffy coats of normal blood bank donors by ultrafiltration and lyophilization, and it ability to influence lymphocyte transformation was assessed by measuring increases in 3H-thymidine uptake into DNA after stimulation with purified protein derivative of tuberculin and/or streptokinase-streptodornase in 23 experiments with cord blood and 15 adult controls. TF stimulated cord blood lymphocytes nonspecifically when added to the medium alone. In the presence of antigen, TF acted in a synergistic, antigen-dependent way with either amplifying or inhibitory effects in adult good responders. These effects were negligible in adult low responders and in cord blood. From our observations no specific conversion of lymphocytes by TF in vitro can be deduced.