A total of 917 cumulus-oocyte complexes were used in this experiment. In Group VI the cumulus-oocyte complexes were matured and fertilized in vitro. After an initial 48 hours culture in microdrops they were transferred to a granulosa cell monolayer. Development to morulae/blastocysts was evaluated 8 days after in-vitro fertilization. The treatment for Group I consisted of 50 ng/ml insulin-like growth factor I (IGF-I) added to in-vitro maturation media. Group II received IGF-I to in-vitro culture on the granulosa cell monolayer. In Group III of IGF-I was added to in-vitro maturation media and in-vitro culture on the granulosa cell monolayer. Group IV received IGF-I to in-vitro culture and was cultured the whole time without a granulosa cell monolayer. Group V received IGF-I to in-vitro maturation and were cultured in-vitro without a granulosa cell monolayer. After supplementation of in-vitro maturation media with IGF-I, 52, 49 and 51% of the cumulus-oocyte complexes in Groups I, III and V did not show cumulus expansion, whereas in Group VI (control) only 1% of the cumulus-oocyte complexes did not expand (P<0.01); in Groups II and IV 6 and 0%, respectively, did not expand. The cleavage rates in the five treatment groups were similar to those of the control group (55 to 71% vs 65%). In Group III, significantly more cleaved oocytes developed to morulae/blastocysts than in the control group (31 and 16%, respectively; P<0.05). In Groups IV and V only 2 and 5%, respectively, of the cleaved oocytes developed to morulae and blastocysts. These results demonstrate thatcumulus expansion does not permit the prediction of subsequent cleavage rates. Furthermore, addition of IGF-I to the in-vitro maturation and in-vitro culture media can increase the number of morulae and blastocysts, but addition of IGF-I could not replace co-culture with granulosa cells.