Colony Stimulating Factors (CSF-1, -2, and -3) are found in the reproductive tract and have been shown to have beneficial effects when added to embryo culture media in a variety of species including humans. CSFs are also found in the ovarian follicle suggesting they may have a role in oocyte maturation. We have recently shown that CSF2 can improve in vitro oocyte maturation, however the role of CSF-1 and CSF-3 has not been determined. The present study was undertaken to determine if CSF-1 - also known as Macrophage-Colony Stimulating Factor (M-CSF) can improve in vitro oocyte maturation (IVM). To determine this a dose response experiment using doses shown previously to improve embryo development when added to embryo culture media, was undertaken and preimplantation embryo development examined. Female mice were injected with eCG and cumulus oocyte complexes (COCs) aspirated from large antral follicles 46-48 h post injection. COCs were matured with either 0, 2 or 10ng/ml of CSF-1. Following fertilization, presumptive zygotes were cultured and embryo development examined, with day 5 blastocyst cell numbers determined by differential staining. No difference was seen between the control, 2 and 10 ng /ml groups in cleavage rate (mean 94% vs 94% vs 90% respectively) or day 4 morula rates (22 vs 18.5 vs 19.7), and day 5 blastocyst rate (68.6% vs 74.6% vs 74%). There was also no difference in day 5 blastocyst inner cell mass cell numbers (16.6 vs 17.7 vs 17.7), trophectoderm (41.4 vs 41.4 vs 43.3) or total cell numbers between the three groups. In conclusion our results suggest that adding CSF-1 during IVM does not improve oocyte developmental competence.