Because certain (2-chloroethyl)triazenes and (2-haloethyl)nitrosoureas have high antineoplastic activity, 2-chloroethyl and 2-fluoroethyl sulfonates were prepared to try to develop additional types of 2-haloethylating agents. In this initial study, it was demonstrated that antineoplastic activity much superior to that of the prototype, 2-chloroethyl methanesulfonate, could be found among 2-chloroethyl sulfonates. Among a variety of 2-chloroethyl alkane- and arenesulfonates, several substituted methanesulfonates displayed significant activity against P388 leukemia in mice; the chloromethanesulfonate showed high activity (T/C = 218%). None of the arenesulfonates were active in this test.