When alpha- momorcharin was injected intraperitoneally (0.2 mg/25 g body weight) into pregnant mice on Days 1-3 of pregnancy, over 50% of the mice failed to support an implantation. In-vitro study of the effects of the protein on preimplantation embryos showed that the protein did not significantly disturb embryonic development from the 2-cell to compacting morula stage except when high concentrations (greater than or equal to 0.5 microgram/ml) of protein were present. In many embryos, compaction of blastomeres was incomplete and subsequent blastocyst formation was impaired. Other protein-treated embryos that formed compacted morulae and early blastocysts later showed decompaction and degenerated. The protein-treated embryos generally had fewer numbers of cells because cell division beyond the morula stage was impaired. The poor development of morulae may be the cause of inhibition of early pregnancy in the mouse by alpha- momorcharin .