STAR, also known as steroidogenic acute regulatory protein, appears to mediate the rapid increase in pregnenolone synthesis stimulated by tropic hormones. It increases pregnenolone synthesis more than four-fold and a major STAR transcript of 1.6 kb is found in ovary and testis. During ongoing growth and differentiation of the follicle of the ovary, the immunoreactivity of it tends to shift from the granulosa cells of early antral follicles to the theca cell layers in the adult. Recombinant human STAR protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.