Summary. Mouse embryos at the 1-cell stage were cultured in various concentrations of mycophenolic acid and xanthine. Mycophenolic acid at 10 and 2·5 μg/ml completely inhibited development to the morula and blastocyst stages, respectively. The addition of xanthine reversed the inhibitory effects of mycophenolic acid on the embryos but the reversal effect depended on the concentration of xanthine. Development was normal with a xanthine concentration of 25 μg/ml, even when an inhibitory concentration of mycophenolic acid was present. Embryo transfer experiments showed that the blastocysts formed in vitro in the presence of mycophenolic acid + xanthine could implant in foster mothers and develop normally to fetuses.