Echinomycin is a quinoxaline antibiotic that was originally isolated from Streptomyces echinatus. Based on its antitumor activity against two i.p. implanted murine tumors, the B16 melanoma, and the P388 leukemia, it was brought into clinical trials by the National Cancer Institute. Recent studies on its cytotoxic action have related its antitumor activity with its ability to bifunctionally intercalate with double stranded DNA. Toxicologic studies were carried out in CDF1 mice and beagle dogs using intravenous injections. For the mice studies the dose ranges were 288-692 mcg/kg (864-2076 mcg/m2) by single bolus, and 112-254 mcg/kg/day (336-762 mcg/m2/day) for five consecutive days. In the dog, dose ranges studied were 8.9-89.4 mcg/kg (178-1788 mcg/m2) by single bolus, and 3.4-33.5 mcg/kg/day (68-670 mcg/m2/day) for five consecutive days. The major toxic effects were found in the gastrointestinal, hepatic, and lymphoreticular systems. These were reversible at all but the highest dose, in dogs that had been treated for five consecutive days. Phase I clinical trials using various intravenous schedules were sponsored by the National Cancer Institute. Nausea, vomiting, reversible liver enzyme abnormalities, and allergic reactions were the most common toxicities encountered. Based on results from these studies, the National Cancer Institute has recently begun phase II trials in a broad range of diseases. These trials will further characterize echinomycin's toxic effects and its antitumor activity.