Suppressor T lymphocytes play a major regulatory role in the function of the immune system. Since the discovery of histamine (H2) receptors on the surface of these immune cells, studies have demonstrated that cimetidine diminishes the effect of suppressor T cells in both cellular- and humoral-mediated (antibody) immune reactions. Preliminary clinical results demonstrate that cimetidine has a beneficial effect on a variety of immune functions. An antineoplastic effect among tumor-bearing animals has also been found, suggesting the drug may be effective against certain human cancers, probably as an immune potentiator. Cimetidine has been used in the treatment of human cancer in combination with interferon or coumarin and as a single agent. Modest tumor response rates have been observed. The finding that cimetidine accelerates healing of herpesvirus infections is intriguing. These encouraging, albeit preliminary, results emphasize the need for additional studies on the drug's antiviral properties.