AbstractA general method for the interpretation at the molecular level of the genetic products of complex‐acting mutagens is described, using ultraviolet (UV) mutagenesis of phage T4 as an example. The roles of different kinds of intracistronic suppressors in reversion is described. In attempts to modify the course of UV mutagenesis, both caffeine and photoreactivation were found not detectably to affect induced mutation rates, nor was either treatment mutagenic by itself. The mutagenicity of proflavin was shown by test with F‐UdR to be independent of replication, and was also shown not to depend upon the special glucosylation of phage DNA. In experiments testing for a possible role of recombination, proflavin mutagenesis was not augmented by recombination‐stimulating doses of UV. However, the induced mutants did appear to arise in recombinational heterozygotes. Induced mutant clones did not contain the two predicted products of unequal crossing‐over. Possible mechanisms of proflavin mutagenesis were con