We shall continue determinations of changes in DNA base sequence in forward mutagenesis of lambda phage. Mutagens to be studied include: ultraviolet light, ionizing radiations, benzpyrene, nitrous acid, heat, acridine derivatives, and others. We shall also study the action of mutagens on known base sequences by meaasuring induced reversion to wild type of phage mutants which have been sequenced. We shall interpret the results in terms of the lesions causing the mutations, and the enzymatic mechanisms involved. Because of correlations between mutagenicity and carcinogenicity of various agents, and the recent identification of cellular transforming genes, the results should also be relevant to carcinogenesis.