Calf serum (CS) is a common supplement used in cell culture. It has been suggested that CS contains substances protecting cells against apoptosis. To examine whether a culture system including CS is appropriate for studying apoptosis in bovine luteal cells, we examined the influence of CS on the expression of Fas, bcl-2 and bax gene. Since progesterone (P(4)) is known to be an anti-apoptotic factor in bovine luteal cells, the present study was carried out to examine the P(4) effect on apoptosis. Bovine mid-luteal cells were exposed to Fas ligand (Fas L) in the presence or in the absence of P(4) antagonist (onapristone, OP) in a basal medium (BM) containing 5% CS (BM-CS) or BM containing 0.1% BSA (BM-BSA). Although Fas L alone, OP alone or Fas L plus OP did not show any cytotoxic effect on the cells cultured in BM-CS, administration of OP or OP in combination with Fas L resulted in the killing of 30% and 55% of the cells cultured in BM-BSA medium, respectively (p<0.05). Concomitantly, CS inhibited bax mRNA expression and stimulated bcl-2 expression in the cells (p<0.05). Moreover, in the cells cultured with BM-CS, Fas mRNA expression was smaller than that of cells incubated in BM-BSA medium (p<0.05). The overall results suggest that CS suppressed Fas-mediated cell death in cultured bovine luteal cells by promoting the ratio of bcl-2 to bax expression and by inhibiting Fas expression. Therefore, it may be suggested that CS contains such anti-apoptotic substances (growth factors) amplifying the cell survival pathways in the bovine corpus luteum (CL) in vitro.