SummarySeveral homologues andN‐substituted derivatives of ethyl carbamate have been examined for tumour‐initiating properties by injection into mice followed by promoting treatment with croton oil. The LD50of the compounds was also determined. The tumour‐initiating potencies were ranked in descending order ethyl carbamate, ethylN‐methylcarbamate, ethylN‐ethylcarbamate butyl carbamate; n‐propyl and isopropyl carbamates with doubtful effect; and ethylN‐n‐propylcarbamate, ethylN,N‐dimethylcarbamate, ethylN,N‐diethyl‐ carbamate and methyl carbamate without significant effect. The LD50of homologous series decreased exponentially with increasing molecular weight. It was deduced that the tumour‐initiating properties do not vary with the toxic properties of the carbamates nor with their molecular weight but appear to depend on structural features of the molecule, namely the nature of the ester group, the presence of a free hydrogen in the amide position and the nature of a single substituent in the amide position.