Monocytes derived from the peripheral blood of psoriatic patients demonstrated a significantly higher phagocytic capacity (36 to 40%) for both 125I-labeled Shigella flexneri and 125I-labeled Staphylococcus albus compared with monocytes from healthy subjects. Monocytes from psoriatic patients showed a 2-to-4fold increase in bactericidal capacity against S. albus when compared with normal monocytes. However, the bactericidal capacity of monocytes from diphylline-treated patients did not differ from that of the control subjects. The antibody-dependent cellular cytotoxicity (ADCC) activity against EL4 tumor cells was found to be similar in both psoriatic patients and control subjects. It is postulated that these abnormalities of monocyte function in psoriasis are caused by a decreased cAMP/cGMP ratio similar to the decreased cAMP/cGMP ratio found in the lesional epidermis of this disease. It seems therefore, that the psoriatic abnormality is not confined to only one type of cell, the epidermal cell.