The mechanisms of luteal regression and rescue in women are unknown but forms of programmed cell death may be involved. The proto-oncogene bcl-2 is an important inhibitor of apoptosis but has not previously been described in the human corpus luteum. Immunohistochemical localization of bcl-2 protein was investigated in human corpora lutea obtained from women undergoing surgery during endocrine monitored menstrual cycles as well as from women who had been treated with human chorionic gonadotrophin (HCG) to prolong the luteal phase. Bcl-2 was found to be localized in granulosa-lutein, theca-lutein (as identified by co-localization of P450(17)alpha-hydroxylase) and the endothelial cells around some blood vessels. Immunoblotting demonstrated the presence of a single band of approximately MW 26 kDa. There was no apparent change in either the intensity of immunostaining or the histological localization during the normal luteal phase or following treatment with human chorionic gonadotrophin. The product of the proto-oncogene bcl-2 is present in the human corpus luteum. It is unlikely that bcl-2 expression alone is responsible for prolongation of the lifespan of the corpus luteum in early pregnancy although it is possible that the action of the bcl-2 gene present is modified by changes in other members of the bcl-2 family.