The sensitivity of phage T7 to epoxides and freshly prepared solutions of di(2-chloroethyl) methylamine (HN2) was identical with that of T2. T7, however, proved considerably the more sensitive to ethylenimine and to aged solutions of HN2. It was considered that this was due to the cationic nature of these latter agents affecting the rate of penetration into the phage heads, and that the susceptibility of T2 and resistance of T7 to osmotic shock was a parallel phenomenon. Confirmation was afforded by the fact that a strain of T4 sensitive to osmotic shock behaved like T2, and a resistant strain of T4 like T7. These results, together with others previously reported, are believed to offer very strong evidence that inactivation of bacteriophage by alkylating agents derives from reaction with the deoxyribonucleic acid moiety, probably leading to a failure of the injection process.