The ability to mature human primordial follicles in vitro would assist fertility restoration. However, the signals initiating growth of primordial follicles are unknown. Growth factors such as leukaemia inhibitory factor (LIF) may play a role in this process. To investigate the expression of LIF and its receptor in early developing follicles, nine ovarian samples from adolescents/adults aged 13-43 years and 23 ovaries from human fetuses aged 19-33 gestational weeks were immediately fixed or frozen. The fixed samples were prepared for a study of immunocytochemical staining of LIF and its two receptor units (LIF-R and gp 130). mRNA was extracted from the frozen ovarian samples, and the expression of LIF, LIF-R and gp 130 was investigated by RT-PCR. Products were resolved by 10% polyacrylamide gel electrophoresis and image analysis. There was strong to moderate immunocytochemical staining for LIF and LIF-R in oocytes from the primordial follicular stages onwards, and very weak to moderate staining for gp 130. LIF-R was also detected in granulosa cells of primary and secondary follicles from adolescents/adults. Transcripts of LIF, LIF-R and gp 130 RNA were identified by RT-PCR in all samples. The immunocytochemical staining and mRNA expression of LIF and its receptor are consistent with the concept that LIF might be involved in growth initiation of human primordial follicles through its receptor.