Bovine serum albumin has been treated with the following biologically active (radiomimetic) alkylating agents; nitrogen mustards, epoxides, esters of methane sulphonic acid and an ethyleneimine. The extent of reaction with the different reactive side chains of the protein has been determined; all the agents esterify carboxyl groups and react with histidine although in every case only a fraction were accessible to the reagent. Combination with amino groups was only appreciable with the epoxides and none of the reagents reacted with tyrosine. The most reactive groups towards all the alkylating agents were the —SH groups in denatured egg albumin. In the native protein these were not available for reaction.