The lymphocyte proliferation of 15 psoriatic patients was sutdied after stimulation with tuberculin, trichophytin, varidase, concanavalin A (Con A), and pokeweek mitogen (PMW) during the first and the eighth PUVA treatment, 2 h after oral intake of 8-methoxypsoralen (8-MOP), immediately after UVA-irradiation, and 24 h later. There was a significant decrease of the lymphocyte proliferation, after stimulation with Con A (P less than 0.05), trichophytin (P less than 0.01) and varidase (P less than 0.05), not with PWM and tuberculin, during the first PUVA treatment by 8-MOP alone. This decrease was not significantly changed by the following UVA irradiation and could not be demonstrated after 24 h. No dark effect of 8-MOP on the lymphocyte proliferation could be seen during the eighth treatment. 8-MOP seems to influence lymphocyte proliferation in the dark by interfering with the cell surface, not with the nucleus. The disappearence of this effect during PUVA therapy could be explained by a modification on the surface structure of lymphocytes.