We evaluated the effects of 2.5xlO-10 M or 5xlO-10 M concentrations of human pituitary prolactin (pPRL), human recombinant non-glycosylated (NG-PRL) and glycosylated (GL-PRL) prolactin on the proliferation of normal human lymphocytes with or without coactivation by interleukin-2 (IL-2). None of the PRL forms alone affected the lymphocyte proliferation in a serum-free medium, however, the stimulatory activity of IL-2 was significantly potentiated with all 3 PRL variants. Since the 5xlO~10 M concentrations of individual PRLs exerted the same effects, this result suggests that GL-PRL in primary lymphocyte culture is not a less mitogenic form, if sufficient amounts of IL-2 are available.