E~~a~ellular virus particles to produce many mutations with rather little killing (LOVELESS 1958). The induced mutations may be delayed by several generations (GREEN and KRIEG 1961 ) . The mutagen reacts with three of the four bases naturally occurring in DNA (REINER and ZAMENHOF 1957; BROOKES and LAWLEY 1960, 1961a, 1962; PAL 1962), yet there is reason to suspect that it might be quite specific as to the type of base pair changes it usually induces. The purpose of this investigation was to compare the frequencies of EMS-induced mutations at various sites within the rII region of bacteriophage T4, and to test the notion that predominantly one type of base pair substitution might be induced. Four years ago in this laboratory, during a search for strongly EMS-revertible point mutants. D. M. GREEN found that some AP mutants [mutants which had been produced from standard I+ phage by AP) were quite EMS-revertible, but that none of the 18 EMS-produced mutants he examined were induced by EMS to revert to a comparable extent. This prompted the working hypothesis that one of the base pair transitions-either GC to AT or AT to GC-was much more strongly inducible by EMS than was the other transition. It was decided to test this possible specificity by checking a group of EMS mutants for EMS-induced reversion by a more sensitive test than had been previously employed, and to test similarly additional base analog and proflavin mutants. While this work was in progress, the results of another investigation with similar intent were reported (BAUTZ and FREESE 1960; FREESE 1961). The results differed in several respects for mutants that were included in both investigations; most striking was that the other group of workers found much less induced reversion. Althouyh the other workers used EES, the chemistry of the two ethylating agents would be expected to be very similar. It seemed that the difference must have arisen from the different techniques employed. This possibility led to an expansion of our work, and the chief reason for the conflicting resultswhich will be discussed in context in this paper-has now been clarified. We shall Operated by Union Carbide Corporation for the U. S. Atomic Energy Commission. 2 The abbreviations used are: G, guanine; C, cytosine or 5-hydroxymethylcytosine; A, adenine; T, thymine; EMS, ethyl methanesulfonate; EES, ethyl ethanesulfonate; AP, E aminopurine; BU, 5-bromouracil or 5-bromodeoxyuridine; P. proflavine; and DNA, deoxyribonucleic acid.