We have investigated the biological characteristics of an immortalized granulosa cell line (COV434), which may be used to study follicular and oocyte maturation in vitro. Granulosa cell function was defined as consisting of three distinct properties: (i) production of 17beta-oestradiol in response to follicle stimulating hormone (FSH); (ii) presence of specific molecular markers of apoptosis enabling the induction of follicular atresia; and (iii) capacity to form intercellular connections with cells surrounding an oocyte. The addition of FSH to the culture medium supplemented with 10% fetal calf serum and 4-androstene-3,17-dione resulted in proliferation of the COV434 granulosa cells and in an increased synthesis of 17beta-oestradiol, indicating the presence of the FSH receptor and cytochrome P450 aromatase in these cells. The receptor for luteinizing hormone (LH) was undetectable. Similar expression of various apoptosis-associated genes was found in COV434 granulosa cells and in granulosa cells of patients stimulated with gonadotrophins for in-vitro fertilization, thus indicating that the immortalized COV434 granulosa cells were able to sustain apoptosis. Multiple intercellular connections were formed during co-culture of COV434 granulosa cells with cumulus cells containing an immature oocyte but not with cumulus cells devoid of an oocyte. Detailed morphological analysis of the intercellular connections with scanning electron microscopy and confocal light microscopy demonstrated the presence of long slender structures. It is concluded that the immortalized human granulosa cell line COV434 may be useful for experimental studies on follicular development.