DeveloPment Promoting Factor-1 (DPF-1) was derived from the rabbit oviductmucosa epithelial cells. fertilized eggs were treated with DPF-1 and it was adminis-tered to female mice in order to investigate its effect on embryonic development and implantation. The results of experiment I indicated that of the 82 fertilized eggs cul-tured in the absence of DPF-1, only 9 were normally divided to the morula stage onday4. By contrast, in the medium containing DPF-1 109(90% ) of 121 embryos weredeveloped to the blastocyst stage. In eoperiment Ⅱ, the developed blastocysts weretransferred into the uterus of 3-day pseudopregnant DPF-1-treated or control recipi-ents. Three different combinations of intrauterine transfer were performed: the DPF-1 treated embryos transferred to the control reclpients(DPF- 1→C),the control embryosto the DPF- 1 treated recipients(C→DPF- 1), and the control embryos to the control re-cipients (C→C). The implantation rate was assessed on day 18 after transfer. Therewere significant differences in pregnancy rate per mouse between DPF-1→C and C→DPF- 1 or control groups(C→C). The embryo implantation rate was low in C→DPF-1 (26. 0%, 75/288) and C→ C (25 3%, 73/288) as compared with DPF-1→C(61. 8%, 178/288). These results suggest that the major site of the DPF-1 actionduring early pregnancy is the embryo itself. DPF-1 enables embryos to develop, buthas no obvious direct effect on uterine receptivity. DeveloPment Promoting Factor-1 (DPF-1) was derived from the rabbit oviductmucosa epithelial cells. Fertilized eggs were treated with DPF-1 and it was adminis-tered to female mice in order to investigate its effect on embryonic development and implantation. of experiment I indicated that of the 82 fertilized eggs cul-tured in the absence of DPF-1, only 9 were normally divided into the morula stage on day 4. By contrast, in medium containing DPF-1 109 (90%) of 121 embryos were eveloped to the blastocyst stage. In eoperiment II, the developed blastocysts were transferred into the uterus of 3-day pseudopregnant DPF-1-treated or control recipi-ents. Three different combinations of intrauterine transfer were performed: the DPF-1 treated embryos transferred to the control reclpients (DPF-1 → C), the control embryosto the DPF-1 treated recipients (C → DPF-1), and the control embryos to the control re-cipients day 18 after transfer. Th erewere significant differences in pregnancy rate per mouse between DPF-1 → C and C → DPF-1 or control groups (C → C). The embryo implantation rate was low in C → DPF-1 (26.0%, 75/288 ) and C → C (25 3%, 73/288) as compared with DPF-1 → C (61.8%, 178/288). These results suggest that the major site of the DPF-1 actionduring early pregnancy is the embryo itself. DPF-1 enables embryos to develop, buthas no obvious direct effect on uterine receptivity.