The evidence for regarding DNA as a “genetic substance” in viruses, bacteria and higher organisms is reviewed, and information obtained from studies in chemical mutagenesis presented. Although certain substances are capable of initiating mutation in micro-organisms which may not have this property for higher organisms, and vice versa, the universality of action of certain alkylating agents as mutagens is considered to indicate a common mechanism of action. The biological results and the known chemistry of such alkylating agents warrant the assumption, which should not be difficult of verification, that this mechanism is based upon primary reactions with DNA. The proposition of any model purporting to explain the occurrence of mutation as a consequence of alkylation of DNA must take account of five questions posed.