The effects of Azathioprine (AZA) on the carcinogenesis with 3-Methylcholanthrene (MC) are described. When AZA is given before MC, there is a shortened tumor latency period. When AZA treatment followed MC injection a lengthened latency period was observed. When AZA treatment is given both before and after MC injection, intermediate results were observed. It is concluded that the action of AZA treatment on tumor development depends on the time relationship between this treatment and the injection of the carcinogen. In particular, it is suggested that to obtain an enhancing effect on tumor development, probably by immunosuppression, AZA must be effective at the time or shortly after administration of the carcinogen. When AZA is administered later, an inhibition of tumor development, probably a cytostatic effect becomes prominent.