Several naturally occurring peptides in bovine milk were characterized by tandem mass spectrometry and Edman degradation. Chromatograms of peptide fractions (passed through an ultra-filtration membrane, nominal molecular weight limit 3000) prepared from colostrum (collected immediately after parturition) and transitional milk (collected 5 days postpartum) showed that they were almost identical. In total, six peptides, αs1-CN (f16-23) (RPKHPIKH), αs1-CN (f16-24) (RPKHPIKHQ), αs1-CN (f17-25) (PKHPIKHQG), αs1-CN (f46-52) (VFGKEKV), αs1-CN (f94-105) (HIQKEDVPSER), and β-CN (f121-128) (HKEMPFPK), were identified. One of the major peptides, the N-terminal fragment of αs1-casein, varied structurally during early lactation: αs1-CN (f17-25) (PKHPIKHQG) and αs1-CN (f16-23) (RPKHPIKH)/αs1-CN (f16-24) (RPKHPIKHQ) were found in colostrum and transitional milk, respectively. A chemically synthesized peptide, αs1-CN (f16-23) (RPKHPIKH), inhibited apoptosis of bovine granulosa cells induced by serum-free conditions in a dose-dependent manner, in consequence of caspase-3 and caspase-9 suppressions. The physiological function of the peptide remains unclear, but it may have potential use as pharmaceutical agent and as an anti-apoptotic agent in cell culture medium. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.