Using mononuclear cells (MNC) of patients suffering from psoriasis for production of colony-stimulating factors (CSF) an abnormal colony formation in bone marrow of healthy volunteers was detected (colony-forming assay): Number of colonies per 10(5) bone marrow cells after cultivation with 10(6) MNC of patients suffering from psoriasis vulgaris: 65 +/- 14 (n = 8); psoriasis arthropathica: 63 +/- 11 (n = 8). Control values (10(5) bone marrow cells plus 10(6) MNC of healthy donors): 116 +/- 23 (n = 10). Statistical differences: psoriasis vulgaris: p less than 0.001; psoriasis arthropathica: p less than 0.001. The mechanisms responsible for these abnormalities in hematopoiesis are still unknown. They may be of pathogenic significance in psoriasis.