L-Glutamine has been shown to play an important role during in vitro culture of mammalian embryos. However, it is easily decomposed into ammonium, which is believed to have deleterious effects on preimplantation embryos. In this study, we assessed prospectively the developmental competence of human embryos cultured in medium containing L-glutamine or a novel stable glutamine derivative and vitamins. The subjects of this study were 41 women who underwent IVF/ET treatment from September to November 2006 and from whom 6 or more oocytes were retrieved. Sibling oocytes were randomly divided into EA/BA (EmbryoAssyst and BlastAssyst containing a novel stable glutamine derivative and vitamins), and BAS groups (BlastAssyst system containing L-glutamine). There was no difference in pronuclear formation rate between EA/BA and BAS (74 vs. 69%). The blastulation rates of embryos based on the number of zygotes cultured in EA/BA on Days 5 (Day 0=insemination, 54%) and 6 (63%) were significantly higher (P<0.05) than those cultured in BAS (Day 5: 33% and Day 6: 45%, respectively). The present data indicate that a medium containing a novel stable glutamine derivative and vitamins supports the developmental competence of human embryos.