The addition of GM-CSF to embryo culture media has been shown to improve embryo development in a range of species including humans, however, its effects on in vitro oocyte maturation have not been widely examined. The present study was undertaken to determine whether the addition of GM-CSF to mouse oocyte maturation media could increase preimplantation embryo development. C57Bl6 x CBA F1 female mice age 21-23 days were injected IP with dose 0.1ml of eCG and cumulus oocyte complexes (COCs) aspirated from large antral follicles 46-48 h post injection. Ten COCs were cultured per 50μL drop in bicarbonate-buffered α-MEM containing 3 mg/ml BSA and 1 mg/ml Fetuin and 5 mIU/mL FSH plus 0, 2 or 10ng/ml of GM-CSF. Following fertilization presumptive zygotes were cultured in Cleave Media (Cook Medical) and cleavage rate and Day 4 and 5 blastocyst rates determined. Blastocysts were differentially stained on day 5 to determine inner cell mass, trophectoderm, and total cell number. The experiment was replicated six times with 50-60 oocytes per group in each. Data was analysed using one-way ANOVA and Turkey post hoc test. The addition of GM-CSF during oocyte maturation had no effect on cumulus expansion and cleavage rate. The number of blastocysts present on Day 4 was not different between the groups. The number of blastocysts present on Day 5 and the proportion of these that were expanded and hatched were not different between the groups. However, the addition of 10 ng/ml of GM-CSF significantly increased blastocyst inner cell mass and total cell numbers compared to control by 52.1% (18.5 vs 12.2%) and 32.9% (55.2 vs 41%) respectively (P<0.01). Further studies are warranted to confirm our initial findings as well as to determine whether these increases implantation and live birth rates.