The effect of cysteine on the immunosuppressive activity of busulphan, cyclophosphamide and nitrogen mustard.

The effect of cysteine on the immunosuppressive activity of three alkylating agents was tested. Cysteine strongly inhibited the ability of cyclophosphamide and nitrogen mustard to depress the direct plaque-forming cell response of mice to sheep red cells. In contrast, the activity of busulphan was usually potentiated by administration of cysteine. This throws doubt on the hypothesis that busulphan exerts its cytotoxic effects by alkylating thiol groups.