The cytolyticClostridium perfringensdelta toxin lyses selectively cells which express ganglioside GM2. In this study, we investigated whether delta toxin can be used to characterize GM2 on tumor cell membranes and as an antitumor agent. The sensitivity to lysis by delta toxin of various murine and human malignant cell lines and also normal tissues was quantified using a51Cr-release assay. The cytotoxicity titers were correlated with the125I-labeled toxin binding capacity of sensitive and insensitive cells. Seven of eight human melanomas tested were lysed by the toxin and, of these, four were very sensitive (cytotoxicity titers below 12 ng of toxin). All neuroblastomas, gliomas and the retinoblastoma tested were lysed with 3–18 ng of toxin. Three of six carcinomas and one of two sarcomas were also very sensitive (cytotoxicity titers 0.6–15 ng) whereas leukemias and lymphoma cells were insensitive. Normal human tissues were insensitive (erythrocytes, skin fibroblasts) or poorly sensitive (brain, lung, spleen). Thein vivoantitumor activity of delta toxin was tested in tumor-bearing mice. Daily intra-tumor injections of 0.5–1 mg of toxin for 4–5 days in carcinoma Me180- and melanoma A375-bearing nude mice, and neuroblastoma C1300-bearing A/J mice significantly inhibited tumor growth for 12–36 days. Intravenous administration of 100 ng of toxin per day for 5 days in Me180- bearing nude mice and C1300-bearing A/J mice gave significant inhibition of tumor growth only during the treatment period, and 10 injections of the same dose of toxin had no significant effect on SK-MEL28, a tumor lacking GM2.