Phenothiazines and its related compounds have shown diverse biological activities including psychotropic, anticancer and other pharmacological activities. Recent studies have suggested the possible interactions between phenothiazines and their physiological targets or potential receptors. New types of phenothiazine, such as half-mustard type phenothiazines and benzo[a]phenothiazines, have been synthesized. These compounds stimulated T-cell blast formation, natural killer cell activity (possibly via activation of monocytes and macrophages) and antibody-dependent cellular cytotoxicity of human peripheral blood mononuclear cells, and showed cytotoxicity against several human cancer cell lines. Benzo[a]phenothiazines induced monocytic differentiation and apoptotic cell death (characterized by internucleosomal DNA fragmentation) in human myelogenous leukemic cell lines, but not in other cancer cell lines. These compounds also induced antimicrobial activity in vivo, possibly by host-mediated immunopotentia tion. On the other hand, phenothiazines did not induce such immunopotentiation activity, but showed direct antibacterial activity in vitro. There was positive relation between their radical intensity and biological activities. These compounds did not show any apparent mutagenic activity, but rather be antimutagenic. These data suggest their possible applicability of half-mustard type phenothiazines and benzo[a]phenothiazines for cancer chemotherapy.