Pretreatment with disulfiram (DSF, 1000 mg/kg p.o.) 2 h prior to intraperitoneal injection of 1-(2-hydroxyethyl)3-(2-chloroethyl)-3-nitrosourea (HECNU, NSC 29485) reduced the acute toxicity of HECNU by 50% in Sprague--Dawley rats. Thereafter the effect of this additional treatment on the chemotherapeutic activity of HECNU was investigated. After intraperitoneal transplantation of Yoshida sarcoma ascites cells untreated rats had a median survival time of 8 days. The therapeutic response to a single application of HECNU alone or of DSF followed by HECNU was compared. HECNU was injected intravenously at logarithmically increasing doses from 15 to 61.8 mg/kg. The maximum survival time was increased to about 14 days in rats treated with HECNU. Pretreatment with DSF (1000 mg/kg) resulted in identical or slightly higher life expectancies; it thus reduced the toxic side effects of HECNU without inhibiting its antitumor potency.