Experiments were designed to evaluate the ability of rabbit oviduct epithelial cells (ROEC) or ROEC conditioned medium to promote the development of rat eggs fertilized in Vivo or in Vitro. 61.73% of the eggs fertilized in Vitro and 73.33% of the eggs fertilized in Vivo cocultured with ROEC overcame the 2-cell block (Plate I, tables 1 and 2). Similarly, 67.99% of the eggs fertilized in Vitro cultured in ROEC conditioned medium developed over the 2-cell stage, and nearly half of them developed to morula and blastocysts stage (Table 3). By using 35S-methionine incorporation and autoradiography methods, several polypeptides, with molecular weight of 135 Kd, 68 Kd, 55 Kd, 51 Kd and 44 Kd respectively (Fig. 1), were excreted from rabbit oviduct epithelial cells and found in the ROEC conditioned medium. The possibility of entrance of the ROEC proteins into the developing embryo was tested by determining whether any of the secreted proteins bound to the zona pellucida. The results of iodination by using 125I-containing acylating agent labelling method showed that the 68 Kd and 55 Kd proteins were bound onto the zona pellucida of rat eggs co-cultured with ROEC in vitro for 24 h (Fig. 2). Studies concerning the problem that whether these two secreted proteins are the key factors to promote the development of early embryos and the transition of maternal to zygotic control of embryo development are undertaking.