Mice treated with the plant alkaloid tetrandrine have a reduced capacity to mount delayed-type hypersensitivity (DTH) responses. This suppressive effect on DTH was more pronounced when treatment was begun 2 days prior to immunization with sheep erythrocytes. Similar dosage regimens significantly delayed the rejection of cardiac transplants. By contrast, there was significant enhancement of antibody production. These immunomodulatory properties of tetrandrine may account for some of its potential in the treatment of chronic inflammatory diseases, where immunological mechanisms are known to play a major role in pathogenesis.