Abstract: Rubescensine A (Rub A) was shown to be active against murine tumours in vivo. In the present study, a comparative observation was carried out in 7 lines of human cancer cells, which were exposed in vitro for 1 h to a range of concentrations of Rub A and then cultured for 10 days to assess colony-forming ability. The dosesurvival curves obtained in 6 of the cell lines were of threshold-exponential type, while the most resistant cell line (LTEP-78) derived from a squamous cell carcinoma of the lung only gave a wide shoulder over the same range of doses up to 60μg/ml. When the parameters calculated from the curves are compared, a stomach adenocarcinema cell line (MGc80-3) and an esophageal cancer cell line (CaES-17) appear to be relatively sensitive to the drug. The results suggest that an evaluation of Rub A in patients with either of these two types of cancer might be worth while and that a high initial dose might be important for the drug to induce clinical response of tumours.