A proteinase inhibitor, tentatively termed vitelloinhibitor, was purified from yolk of hen's ovarian follicles. It resembled egg-white ovoinhibitor not only in inhibitory spectrum (active for bovine trypsin and bovine chymotrypsin) but also in thermal stability, pH stability, antiserum reactivity and amino-acid composition. However, vitelloinhibitor had different molecular weight from that of ovoinhibitor. An alpha 2-proteinase inhibitor preparation, isolated from laying hen's serum in the present study, was found to exhibit two bands, and the larger one of the latter corresponded to vitelloinhibitor in molecular weight. The partial N-terminal amino-acid sequence of vitelloinhibitor was the same as those of the two components of serum inhibitor and all three agreed with that of ovoinhibitor. Vitelloinhibitor is likely to be an ovoinhibitor analog derived from a serum precursor, which might be the larger component of alpha 2-proteinase inhibitor.