Four adult patients with chronic mucocutaneous candidiasis were studied to establish a possible role for cimetidine as an immunomodulator. These patients had negative baseline in-vivo and in-vitro cell-mediated immune response to candida antigen as measured by intradermal skin tests, lymphocyte transformation, and leukocyte migration inhibitory factor production to cimetidine, 300 mg by mouth, four times daily. Subsequently four of four patients developed strong (greater than 15 mm) intradermal skin test reactions, and two of four patients produced leukocyte migration inhibitory factor to candida antigen. Skin tests and leukocyte migration inhibitory factor production reverted to baseline negative values when repeated 4 weeks after discontinuation of therapy. After 4 additional weeks on cimetidine, four of four patients showed strong positive skin tests and leukocyte migration inhibitory factor production to candida antigen. Lymphocyte transformation was not affected by therapy.