The effects of estrogen on plasma vitellogenin have been studied in the cockerel by immunoprecipitation techniques using an antiserum prepared against the egg yolk phosphoprotein, phosvitin. The antiserum gave precipitin lines of complete identity to phosvitin and to vitellogenin which was isolated from hen plasma by DEAE-cellulose chromatography and by affinity chromatography using anti-phosvitin coupled to Sepharose 4B. The cross-reactivity of vitellogenin and phosvitin adds support to the concept that plasma vitellogenin is the precursor phosphoprotein of egg yolk phosvitin. In the three-week old cockerel, anti-phosvitin produced no detectable immunoprecipitate in the plasma. However, after a single sc injection of diethylstilbestrol (2.5 mg), plasma vitellogenin levels began to increase at 4 h and reached a maximum 20-30 h after hormone administration. The increase in plasma levels of triglyceride paralleled those of vitellogenin. These studies suggest that there is no significant time lag in the estrogenic induction of plasma vitellogenesis in the cockerel, the longer lag periods observed by other investigators may be a function of the sensitivity of the assays used for detecting vitellogenin.