Protein hydrophobicity of 42 native and partially denatured milk and soy protein samples was determined fluorometrically by using three probes, 1-anilino-8-naphthalenesulfonate (ANS), cis-parinarate (CPA) and 1,6-diphenyl hexatriene (DPH), and chromatographically by using Phenyl Sepharose CL-4B (PSC). PSC and ANS hydro- phobicities correlated well to the protein insolubility determined at zero zeta potential, whereas no significant correlation was observed between CPA. hydrophobicity and protein insolubility. When backwards stepwise regression analysis was applied to 189 data of protein insolubility, a significant correlation (P < 0.001) was obtained between PSC hydrophobicity, zeta potential and protein insolubility. It is suggested that the aromatic hydrophobicity, in conjunction with zeta potential, may play a more important role in protein solublity than the aliphatic hydrophobicity.