Sequential chromatography of human follicular fluid (hFF) on Sephadex G-50, Q-Sepharose and finally Cibracon Blue yielded a protein molecule which biologically had a stimulating effect on steroid secretion (SSF) by ovarian granulosa cells in vitro. SDS-gel electrophoresis followed by blotting on PVDF membrane of the biologically active fraction revealed 3 bands of ca. 28, 27 and 24 kDa. The N-terminal sequence of the 3 bands corresponded completely with that of apolipoprotein A-I. In view of this, it was of interest to examine whether human ovary expresses apo A-I. Using specific primers for apo A-I and apo A-I-like protein (ALP; Richardson et al., 1996), we could demonstrate the presence of mRNA for apo A-I as well as ALP in the granulosa cells. The expression of ALP was found to be much higher than that of apo A-I. Like hFF-protein, purified apo A-I could also stimulate steroid secretion by human granulosa cells incubated in the presence of cholesterol. The functional role of ovarian apo A-I/ALP may be speculated as a transport molecule for steroid precursors.