During the next 4 days, EP increased from 5 to about 25% of the total plasma protein. Electrophoretic mobility, peptide mapping, and immunological crossreactivity showed EP to be indistinguishable from a plasma protein in adult females with vitellogenic ovaries. Two major yolk polypeptides, YP1 (120 kDa) and YP2 (29.6 kDa), were precipitated by (NH4)2SO4 from a yolk protein extract.