This study was carried out to determine the effects of oestrous cow serum containing insulin-like growth factor I (IGF-I) and supplementation with recombinant IGF-I on subsequent development of bovine embryos produced in vitro. When culture medium was supplemented with oestrous cow serum containing 34.8 ng endogenous IGF-I ml-1, more embryos (P < 0.01) developed to blastocysts by day 9 and more blastocysts hatched on day 13 after insemination (P < 0.01) than in the control group. The effect of the addition of 10, 50 and 100 ng IGF-I ml-1 to culture media containing oestrous cow serum and granulosa cells was also evaluated. Supplementation with 10 ng IGF-I ml-1 did not improve embryo development at any stage. The addition of 50 and 100 ng IGF-I ml-1 did not affect development during the first three cell divisions. However, on day 7 these groups yielded a higher embryo rate than did the control group. Similarly, the proportion of blastocysts on day 9 was enhanced. The addition of 100 ng IGF-I ml-1 also increased the proportion of blastocysts. These data suggest that IGF-I at high concentrations accelerates the development to the blastocyst stage by shortening the transition from the morula to the blastocyst stage. The addition of 100 ng IGF-I ml-1 increased the proportion of hatched blastocysts on day 13. The addition of oestrous cow serum and IGF-I to TCM 199 free of granulosa cells did not increase the proportion of embryos on day 7. However, the progress to blastocysts and hatched blastocysts on days 9 and 13 was significantly lower (P < 0.05). The addition of IGF-I to culture medium without oestrous cow serum but with granulosa cells resulted in significantly lower embryo development than in the control group or in the group supplemented with oestrous cow serum and IGF-I (P < 0.01). The results support the hypothesis that culture media containing high concentrations of IGF-I combined with oestrous cow serum and granulosa cells can improve the development of embryos produced in vitro.