The pathogenetic mechanisms of psoriasis and psoriatic arthropathy (PsA) are not completely known. Previous studies have suggested the participation of the immune system in this process. In this work we have analyzed several aspects of B and T cells function in peripheral blood of 40 patients with psoriasis (age 44 +/- 15 yr.). Twenty-eight of them had PsA. As a control group we studied 35 healthy subjects aged 36 +/- 14 yr. The B cell function was evaluated through the following tests: a) Serum immunoglobulin (Ig's) levels; b) Number of circulating IgA-, IgG- and IgM-secreting cells; c) Ig's-secreting cells after PWM stimulation; and d) Response to autologous mixed lymphocyte reaction. To evaluate the T cell function, we studied the response to PHA stimulation and its modification by the inhibition of prostaglandin synthesis with indomethacin. In the group of patients, we observed the following abnormalities: a) A significant increase in serum levels of IgG (1,170 +/- 363 mg/dl in patients versus 885 +/- 167 mg/dl in controls, p less than 0.01) and IgA (250 +/- 143 versus 154 +/- 38 mg/dl, p less than 0.05); b) A decreased response of B lymphocytes to PWM, statistically significant for the number of cells secreting every type of Ig's (p less than 0.001); and c) An impaired response to PHA (47,755 +/- 23,129 cpm in patients versus 73,634 +/- 31,085 cpm in controls, p less than 0.001), that partially improved (34% of enhancement) after inhibition of prostaglandin synthesis with indomethacin.(ABSTRACT TRUNCATED AT 250 WORDS)