The strong carcinogens, N-methyl-N-nitrosourea (MNU), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) react with phosphatidylethanolamine (PE) in chicken erythrocyte ghosts or rat kidney cells, whereas a weak carcinogen, methyl methanesulphonate (MMS), does not. The reactions of these agents with commercial PE were also carried out and their reactivities were compared with their carcinogenic potencies. When compared with the amount of PE of the blank, 64%, 18% and 0% of PE were reacted with MNNG, MNU and MMS respectively. This indicated that MNNG is approximately 4 times more reactive than MNU, while MMS does not react with PE, correlating with their tumorigenic potencies on mouse skin. A positive correlation was also observed between the reactivity of a series of N-methyl-N'-aryl-N-nitrosoureas (I-X) to PE and their tumorigenicity. In the case of the nitrosoureas, the reactions proceeded through carbamoylation of the amino group in PE by the isocyanates generated from the agents. Furthermore, many isocyanates also reacted with PE just as the strong carcinogens did. Our present results suggest that both alkylation of DNA and reaction with cell membranes would be required for the formation of transformed benign cells. For progression of the benign to malignant cells, further alkylation of DNA would be required, and the carcinogenic potency of the agents may result from a combination of both reactions.