Abstract: The in vitro effect of minoxidil on normal human lymphocyte activity was studied. At a concentration of 0.1 mg/ml, minoxidil significantly inhibited phytohemagglutinin-dependent lymphocyte DNA synthesis (p < 0.05), and at 0.01 mg/ml, 0.05 mg/ml, and 0.1 mg/ml, it suppressed mitogen-dependent production of leukocyte migration inhibition factor (p < 0.04, p < 0.01, and p < 0.01, respectively), without compromising cell viability. Minoxidil thus appears to exert a suppressive effect on normal human T-lymphocytes in vitro. The physiologic relevance of these findings remains uncertain.