Indicine-N-oxide, a pyrrolizidine derivative, was selected for development because of activity in the murine P388 leukemia cells model. Route and schedule dependency were demonstrated. The antitumor activity of the drug probably is mediated via antimitotic effects and chromosomal damage. The active metabolic species responsible for these antitumor properties is not yet known. The major toxic effect was myelosuppression. Phase I clinical trials arrived at recommended doses for further study. Colon carcinoma was possibly responsive; several tumor types were stable during phase I testing. In a single phase II study in refractory leukemia, there were 3 responses, including 1 complete response, among 7 patients. Phase II studies in all panel tumors are indicated, especially colon carcinoma and leukemias.