Some non-differentiation-induction benzo[a]phenothiazines and mutagenic benz[c]acridines more potently inhibited the mitogen-induced blast transformation of human-peripheral blood lymphocytes than differentiation-induction and non-mutagenic counterparts and phenothiazines. Differential absorption spectrophotometry revealed tight complex formation between these drugs and bacterial endotoxin or mitogens. All of these compounds only slightly affected antibody dependent cellular cytotoxicity and natural killer cell activity, but significantly inhibited the endotoxin-or heat-killed Staphylococcus aureus induced tumor necrosis factor production by human mononuclear cells. Pretreatment of mice with these drugs protected them from lethal E. coli infection. Quantumchemical analysis suggests a correlation between the biological activity of these compounds and some molecular orbital parameters such as the charge at C7, and the ratio of polar/total surface areas.