Chlorpromazine is now emerging as an adjuvant chemotherapeutic agent for the treatment of neoplasia. This was further supported in the present study by the following lines of evidence: it was shown that chlorpromazine causes damage in a series of native nucleic acids, though at somewhat high concentrations. Furthermore, chlorpromazine and caffeine were shown to act synergistically to potentiate the cytogenetic effect of adriamycin on human lymphocytes in vitro and on Ehrlich ascites tumour (EAT) cells in vivo. It is suggested that chlorpromazine alone or in combination with caffeine may exert its cytotoxic effect on normal and neoplastic cells not only indirectly, i.e. by facilitating the intracellular retention of adriamycin, but also directly by intercalating into nucleic acids.