Sterilization of males con in certain circumstances be more efficient than killing as a method for control of insects and perhaps other pests. A number of chemicals (Chemosterilants) show promise of producing sexual sterility in insects without some of the pratical limitations of radiation. The most important compounds are alkylating agents. These have little immediate pharmacological action, but are notable for their selective action against haematopoietic and some other proliferating tissues. A number of alkylating agents have been shown to be mutagens in insects, bacteria, fungi, and higher plants; carcinogens in mammals; and teratogens in insects, birds, and mammals. Some produce sexual sterility, possibly in mammals as well as in insects, at doses too low to produce the other effects. Some have an established reputation as drugs for palliative treatment of leukaemia and other neoplasms. The developmet of insect sterilization as a vector control technique has been handicapped in part by lack of scientific information on the acute and long-term hazards that might be associated with the use of chemosterilants. In this paper the author brings together the available knowledge on the toxicology of the alkylating agents.