The interaction of phosvitins, the polyphosphoproteins of the eggs of egg-laying vertebrates, with ferric chloride was investigated under acidic conditions at iron-to-protein phosphorus ratios ranging up to 10. Phosvitins of which all or nearly all serine residues are phosphorylated (P/ser greater than 0.8) precipitate when titrated with the iron salt. As the total Fe/P ratio reaches the value of about 0.5, precipitation becomes maximal. At Fe/P ratios above 0.5, the Fe(III)-phosvitin complex becomes increasingly soluble. At ratios above 2, solubility is essentially fully restored. Phosvitins with an appreciable portion of their serine residues non-phosphorylated (P/ser less than 0.7) show a different dependence of solubility on the Fe/P ratio. The Fe/P ratios of all precipitated complexes themselves vary within a narrow range between about 0.4 and about 1.0; the total Fe/P ratio is varied between 0 and 10. The results imply that phosvitin iron binding sites are non-uniform and that, overall, phosvitin is capable of accommodating iron in different ways, depending on the relative magnitude of the iron load and the availability of phosphoserine clusters in the phosphoprotein.