Heat-killed sonicated suspensions of malassezia ovalis and Staphylococcus epidermidis were applied to the intact volar surface of the forearm of ten patients with psoriasis and ten control subjects. All ten of the patients with psoriasis reacted to M ovalis, and four patients reacted to S epidermidis. Two of the control subjects responded to M ovalis, and none responded to S epidermidis. Lesions that both clinically and histologically were consistent with those of psoriasis developed in the patients with psoriasis.