Circulating embryotoxic factors could be responsible for reproductive failures observed in patients suffering from recurrent spontaneous abortions (RSA) and endometriosis. The mouse bioassay has been widely used to detect such factors, since sera from these patients inhibit early embryonic development. This bioassay consists in the in-vitro culture of two-cell mouse embryos in the presence of different sera up to the blastocyst stage (72 h of culture). In the present study experiments were performed over long culture times (3-7 days), from two-cell to spreading stages, to determine the in-vitro effect of sera obtained from RSA or endometriosis patients, as well as the effect of interferon (INF)-gamma on embryo development. An embryotoxicity cut-off value of 45% blastocyst formation was established using control sera. When development to the blastocyst stage was considered only 25% of RSA and 20% of endometriosis sera were embryotoxic. However, all RSA sera significantly inhibited hatching (P < 0.05) and spreading stages (P < 0.01). IFN-gamma (10 micrograms/ml) (P < 0.001) did not impair early embryo development, but significantly inhibited blastocyst spreading. These observations suggest that culture to advanced embryonic stages increases the sensitivity of the bioassay and that IFN-gamma alters in-vitro peri-implantation mouse embryo development.