We investigated the in vitro responses to bacterial superantigens of peripheral blood mononuclear cells taken from patients with psoriasis (one arthropathic, two guttate and four chronic plaque type). We also analysed the relationship between the magnitude of the responses of peripheral blood mononuclear cells to bacterial exotoxins and the number of circulating T cells bearing V[~2 and Vl$3 regions. The proliferative response of peripheral blood mononuclear ceils to Staphylococcal enterotoxin B and toxic shock syndrome toxin-1 was significantly higher in patients with active psoriasis than in normal subjects. An improvement in skin eruption was associated with a decrease in the lymphocyte response to one-half or one-third that of the active phase. There was no significant difference between patients with psoriasis and normal subjects in the percentage of V[~2-and V[~3-positive circulating T cells. The percentages of V[~2-positive and V~3-positive cells were not correlated with the levels of responsiveness to toxic shock syndrome toxin-1 and to Staphylococcal enterotoxin B, respectively. These findings suggest that the magnitude of responses of peripheral blood mononuclear cells to bacterial toxins does not depend on the number of T cells reactive with the relevant superantigen, but depends on the extent of skin lesions in psoriasis.