In order to study the effect of insulin-like growth factor (IGF)-I on amino acid transport, granulosa-luteal cells were obtained from stimulated cycles in women undergoing in-vitro fertilization. The cells were precultured for 1-3 days in serum-containing medium. The medium was then changed to serum-free with added IGF-I and/or human chorionic gonadotrophin (HCG). Following 24 h culture, the cellular uptake of [14C]alpha-aminoisobutyric acid AIB (0.1 mM, 0.6 mCi/ml) was studied during a 2-6 h incubation. The results showed that IGF-I (10-100 ng/ml) consistently stimulated AIB uptake to levels which were 44 +/- 7% above the control (n = 11 experiments). The stimulatory effect of IGF-I was abolished by HCG and was reduced by IGF-binding protein. In conjunction with previous findings that IGF-I stimulates the uptake and incorporation of thymidine into DNA, these results suggest that IGF-I is involved in growth of human granulosa-luteal cells.