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Radiation induced heritable changes in a particular character tend to be infrequent in most of the materials which have been studied. In Escherichia coli, ultraviolet induced mutations to lactose nonfermentation approach two per 1,000 surviving cells plated (Newcombe, 1952), and mutations to phage Ti resistance and to streptomycin resistance and dependence occur even more infrequently. A change to galactose negative (on eosin-methylene blue indicator agar) has been induced with exceptionally high frequencies approaching ten per cent in a strain of Saccharomyces cerevisiae by exposure to ultraviolet light. Although the mechanism underlying this change is not yet fully understood, it can be called a mutation in the broad sense, and its existence permits quantitative and qualitative studies of fluctuations in the mutation pattern which otherwise would be difficult or impossible. Attempts to elucidate the nature and mode of origin of the change will be described elsewhere. The present account deals with the dose relationships associated with this mutation. In particular, the proportion of mutant colonies which were sectored was found to decrease with increasing dose over a range of doses which was associated with very little killing (up to 60 per cent). Evidence which bears on the interpretation of this phenomenon will be considered.

RELATIVE FREQUENCIES OF SECTORED AND NONSECTORED MUTANT COLONIES IN YEAST AS A FUNCTION OF ULTRAVIOLET DOSE