This study compares the effects of several sera on the spontaneous neoplastic transformation of C3Hf mouse embryo cells in vitro. These sera would differ in hormonal content and included fetal bovine, mare, stallion, and gelding horse sera. AHempts also were made to diagnose the neoplastic potential of the cells by means of cytologic criteria developed for similar cell culture systems. The 3 different pools of embryo cells used to initiate cell lines were found to vary in their potential for neoplastic transformation. The reason for this variation is still unclear. From results of animal assays and cytologic observations, mare serum, like fetal bovine serum, appeared to delay or inhibit spontaneous neoplastic transformation as compared with the 2 male sera, stallion and gelding. This effect on neoplastic transformation appeared to be unrelated to the growth-stimulating capacity of the sera.