The effect of local instillation of mepacrine hydrochloride (quinacrine hydrochloride U.S.P., Atabrine) was compared with triethylenethiophosphoramide (Thiotepa) applied locally and with pleurocentesis alone. Twentyfive patients with disseminated malignant disease were allocated at random to one of these three treatments. A finding of more than 500 ml effusion within 3 months of instituting treatment was regarded as a failure, and subsequently one of the other treatments was used at random. Sixty-four per cent of the patients treated with mepacrine responded, compared with 27% treated with triethylenethiophosphoramide, and 11% treated with pleurocentesis alone.