The effect of a single high dose of Levamisole (200 mg/M2) on delayed-type hypersensitivity (DTH) in vivo and on lymphocyte blastogenesis to mitogens and antigens in vitro was studied in 26 patients with carcinoma. Similar studies were conducted in 24 control patients. Levamisole had a moderate but significant enhancing effect on DTH to Dermatophytin detectable no earlier than eight hours and still present at 48 hours after the drug administration. A moderate but significant enhancing effect on lymphocyte blastogenesis to mitogens and antigens was also demonstrated during the same time sequence. Further clinical trials with Levamisole should be conducted with more attention paid to schedule of drug administration.