This study was designed to provide further information on mouse and human embryo development in alpha-modified minimum essential medium (alphaMEM). First, we compared the development and implantation potential of murine in-vitro fertilized (IVF) embryos cultured in alphaMEM, in the presence and absence of co-culture cells. No significant difference was observed in blastocyst rate between alphaMEM alone (76.2%) and alphaMEM plus co-culture (79.9%). The percentage of hatched blastocysts was, however, higher with co-culture (47.5 versus 40%, P < 0.01). Transfer of blastocysts to pseudopregnant foster mothers resulted in similar live birth rates (14.9% alphaMEM alone versus 19.8% alphaMEM/co-culture). alphaMEM was also introduced into our clinical IVF programme for culture of human embryos beyond day 3. Spare human embryos were cultured under oil in microdrops of alphaMEM supplemented with 10% synthetic serum substitute. Blastocysts were evaluated for maturity and the presence and organization of the inner cell mass. A total of 206 embryos from 53 IVF patients underwent extended culture. The overall blastocyst rate was 45.1%. An inner cell mass was observed in 76 blastocysts (81.7%). With regard to developmental maturity, approximately 73% of blastocysts that had been frozen were expanding (cavity > 50% embryo volume) or fully expanded. These data suggest that alphaMEM in conjunction with a commercial protein preparation such as Synthetic Serum Substitute may be a good basal medium for culture of human embryos to the blastocyst stage.