Whole casein from bovine origin, the different casein subtypes alpha, beta, and kappa, and the related dephosphorylated proteins were assayed as modulators of soybean lipoxygenase 1 activity and were found to inhibit it. To define the lipoxygenase inhibitory domain, whole casein and beta-casein were digested by proteases (trypsin, clostripain, and subtilisin). The beta-casein tryptic digest and the tryptic and subtilisin digests of whole casein retained their inhibitory properties. The tryptic beta-casein digest was the most potent inhibitor of lipoxygenase activity and was further fractionated by FPLC or HPLC. The collected peptides inhibited the lipoxygenase-catalyzed reaction to different extents. The active fractions were analyzed by ESI-MS, and the sequences of several lipoxygenase inhibitory peptides, corresponding mainly to the C-terminal moiety of beta-casein, were identified.