Chloroquine is known to exacerbate psoriasis. Since immunological stimuli are considered to be important for the pathogenesis of psoriasis, we compared the effects of chloroquine on cell-mediated immunity in 15 healthy control individuals and 15 patients with psoriasis. We employed the spontaneous and phytohemagglutin (PHA)-induced uptake of 3H-thymidine to measure lymphocyte proliferation. Chloroquine was added to the cultures at concentrations ranging from 0.022 to 220 microM. We found that both spontaneous and PHA-driven lymphocyte proliferations were significantly lower in patients with psoriasis (p < 0.002). The spontaneous blastogenesis in both controls and patients remained stable under chloroquine. In PHA-driven cultures in controls, 0.022-2.2 microM chloroquine had no effect, higher concentrations of the drug suppressed proliferation. In patients, 22 microM chloroquine surmounted the suppression of the PHA-induced proliferative response found in controls; moreover, 2.2-0.022 microM chloroquine increased lymphocyte proliferation by > 300% (p < 0.002). Our data indicate that in psoriasis the lower lymphocyte transformation is abnormally stimulated by the addition of pharmacological doses of chloroquine.