The effect of human lactoferrin on the human lymphoblastic T cell line (Jurkat) was tested with regard to proliferation and differentiation. Lactoferrin enhanced cell proliferation in a serum-reduced (1% fetal calf serum) culture. The stimulatory effect of lactoferrin on proliferation depended on the degree of iron saturation but the amplitude of the effect was low, similar to that obtained in the presence of serum transferrin. The proliferation stimulatory effect of lactoferrin was not observed in the presence of 10% fetal calf serum (FCS) in the culture medium. These results suggest that Fe-lactoferrin can substitute for Fe-transferrin during the prolonged culture of cells in a low serum concentration. Iron-saturated lactoferrin was also shown to promote T cell differentiation. Jurkat cells, when exposed to iron-saturated lactoferrin in the presence of 10% FCS, gradually exhibited a decrease in the cell volume, cell surface density of CD71 antigen, the nuclear incorporation of [methyl-3H]thymidine, but an increase of the percentage of cell population in the G0/1 phase of the cell cycle. These modifications were accompanied by the appearance of CD4 antigen at the cell surface. Therefore, in the continuous presence of lactoferrin, proliferating cells slowly enter into quiescence state, undergoing cell differentiation.