26 patients with psoriasis and 23 patients with other dermatoses were treated for 28 days with oral retinoid Ro 10-9359. Intradermal tests to seven recall antigens were carried out before and after therapy. Dinitrochlorobenzene (DNCB) sensitization was started at the initiation of retinoid therapy and challenge tests made after 28 days. There was a significant increase in the reactions to recall antigens in all groups after 28 days’ retinoid therapy. The reaction to DNCB was increased only in the non-psoriatic group compared with controls. These results demonstrate that the retinoid Ro 10-9359 stimulates cell-mediated immunity in vivo.