Vitellogenin, the only phosphoprotein detectable in the plasma of laying hens, is present at an approximate concentration of 1 mg/mL and can be isolated by chromatography on diethylaminoethylcellulose. Vitellogenin has a molecular weight of 235 000--240 000 and contains approximately 3% phosphorus by weight. Evidence that this protein is the precursor of phosvitins includes its ability to act as an acceptor for phosphate with a phosvitin specific kinase, the generation of a peptide similar to phosvitin by trypsinization, and the presence of distinctive peptides of multiple clustered phosphoserine upon partial acid hydrolysis. This partial sequence similarity between phosvitins and vitellogenin has not been previously reported. The phosphorus content and amino acid composition of vitellogenin are consistent with a model which contains two phosvitins and one lipovitellin. The total molecular weights of these proteins (28 000 + 34 000 + 170 000 = 232 000) are close to that of vitellogenin.