The physical and immunological properties of the estrogen-induced protein from rat uterus are compared with those of ovalbumin from chick oviduct. Both proteins are induced by estrogen and are under estrogenic regulation. Electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels and elution behavior on Sephadex G-100 column chromatography indicate that the uterine protein has a molecular weight of about 42,000, similar to that of ovalbumin, and likewise is composed of only one subunit. On isoelectric focusing in polyacrylamide gels, both proteins focus at a pH of 4.6. The uterine protein, in addition, shows a component with an isoelectric point of 5.1. Despite considerable similarity in the physical properties of the uterine protein and ovalbumin, there is no significant crossreactivity of anti-ovalbumin gamma globulin with the uterine protein, implying that these two estrogen-regulated proteins are immunologically distinct.