Cytembena has been shown to undergo a rapid addition reaction with a number of thiol compounds, including glutathione and cysteine, resulting in alkylation of the sulphur. Administration of Cytembena to Yoshida sarcoma cells and to L1210 leukemia cells resulted in a loss of titratable thiol groups within the cells, though the loss of thiol groups caused by pharmacologically active doses of the drug was not sufficient to account, in itself, for the observed toxicity. The addition product of Cytembena and glutathione was isolated and tested for cytotoxicity; it was much less effective than free Cytembena. It is concluded that this reaction acts as a route of detoxification of Cytembena.