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There is now considerable evidence, particularly in maize and in Drosophila, for the existence of factors which influence the rates of mutation of genes.1 2 The interpretations which have been offered are varied and involve, for example, mutator genes, heterochromatin, or even infective, virus-like particles. Although much still remains to be done, the study of these phenomena constitutes one approach to the general problem of the nature of gene action. The present paper is concerned with the evidence for the existence of analogous effects in a micro6rganism, a variant of the well-known K12 strain of Escherichia coli.3 Our recognition of the special properties of this bacterial variant is not without its instructive, even amusing aspects: in the course of an investigation on possible changes in the nutritive requirements of bacteria after selection for drug resistance, we had observed that a number of streptomycin-resistant K12 stocks acquired a need for one or more amino acids before growth ensued. In one series a requirement for phenylalanine was indicated for a number of independent isolates, in a frequency somewhat beyond that expected by chance. To investigate further this apparent association, we then determined the resistance of a stock phenylalanine-requiring K12 auxotroph which had not previously been in contact with streptomycin. At the inoculum dilution used, the culture appeared completely resistant to streptomyein, inocula of 0.1 ml. of full-grown culture growing out in subcVltures containing 100 ,ug. or more streptomycin per milliliter, while other K12 stocks did not grow in drug concentrations beyond 5-10 jug. This appeared to be a verification of our working hypothesis concerning phenylalanine requirement and increased resistance to streptomycin. Further investigation, with greater dilutions of the culture, revealed, however, that we were dealing not with a high level of resistance for the average organism in the culture but rather with an unusually high rate of mutation to resistance, which resulted in an exceptional number of resistant organisms in an otherwise susceptible population. Of perhaps more general interest is our later observation that, as in the Drosophila material, this increased mutation rate obtains toward a considerable variety of agents, although the mutation rate is not increased uniformly to all states and in fact appears to be normal in at least one instaince. Finally, clear evidence has been obtained that the maintenance of this high rate of mutation is in no way related to a nutritional requirement for phenylalanine and that our original hypothesis in this regard, useful as it was in leading to further observations of interest, must be regarded as only a tentative working hypothesis, based on too small a series of initial observations and to be replaced in the light of additional experiments. Cultures.-The relationships of the principal K12 cultures used, which are of importance for the argument belowv, are best understood from the followA,ing chart:'

A Factor (or Mutator Gene) Influencing Mutation Rates in Escherichia Coli