The effect of 8-methoxypsoralen (8-MOP) in the dark on proliferation of phytohaemagglutinin-stimulated human lymphocytes has been examined, using total blood leucocytes, the lymphocyte fraction and the T-lymphocyte subpopulation. 8-MOP (1 microgram mL-1) increased [3H]TdR incorporation and caused a rise in the blastic and mitotic index in all experimental cultures. The lymphocyte proliferation in these experiments was effected by a comitogenic effect of 8-MOP on T-lymphocytes.