This is a preliminary report on the experimental and clinical use of a new alkylating agent (S‐73) which is water soluble, stable to hydrolysis for hours, but with esterase‐susceptible linkages, which results in inactivation of the agent by esterase in the blood. S‐73 is a potent cytotoxic agent when injected near its target but much larger doses are required to produce cytotoxic effects when injected farther away from it, because of rapid inactivation and dilution. In no instance is bone marrow depressed.