Antipsychotic drugs that bind to and inhibit the action of calmodulin also inhibit cellular proliferation. In addition these drugs are cytotoxic to most malignant cells and can augment the antiproliferative and cytotoxic effects of bleomycin. They are attractive candidates for use against tumors of the central nervous system since they readily pass the blood-brain barrier and accumulate in the brain. To identify more active derivatives, we studied the effect of a series of phenothiazines and a group of related compounds alone or in combination with bleomycin against rat glioblastoma cell lines. C6 cells were grown for 24 hours prior to a 48 hour exposure to anti-psychotic drug alone or to an IC20 concentration of antipsychotic drug with bleomycin. Cells were stained with methylene blue and enumerated spectrophotometrically. Eight phenothiazines were found to augment the effect of bleomycin by > or = 3-fold. These included 1-chlorpromazine (3.8x), chlorpromazine (3.2x), 3-chlorpromazine (3.0x), 4-chlorpromazine (3.4x), thiomethylpromazine (3.3x), didesmethylchlorpromazine (11x), fluphenazine (5.5x) and trifluoperazine (3.2x). Structurally similar compounds also having activity included trans-flupenthixol (6.0x), 2-chloroimipramine (6.0x), desipramine (22x), and penfluridol (24x). There was a direct correlation between the antiproliferative effect of anticalmodulin compounds and the ability of these drugs to inhibit the activation of calmodulin-sensitive phosphodiesterase. However, there was no correlation between the inhibition of calmodulin and the augmentation of the antiproliferative activity of bleomycin. Penfluridol, one of the most active compounds, was chosen for further study. It increased the activity of bleomycin against L1210 leukemic cells by 90-fold and MCF-7 human breast cancer cells by 4-fold. The effect of penfluridol in combination with bleomycin was due to increased cytotoxicity as measured by clonogenic assay.