The precipitation of peptides derived from tryptic hydrolysis of caseins was studied as a function of the pH (3.5 to 7.5), ionic strength (from 0 to 1 M), and the chemical nature of the salt [NaCl or (NH4)(2) SO4] at 25 degrees C. Precipitation occurred only in the acidic pH range. The precipitated fraction, analyzed by RP-HPLC, was constituted by a specific hydrophobic peptide group. The nine major peptides were identified as kappa-CN (35-63), alpha(s1)-CN (91-100), alpha(s1)-CN (125-151), alpha(s1)-CN (152-193), beta-CN (49-97), beta-CN (108-169), beta-CN (114-169), beta-CN (184-202), and beta-CN (184-209). The optimum precipitation was found to be at pH 3.5 and 0.25 M NaCl except for beta-CN (184-202) and alpha(s1)-CN (125-151), which required a higher ionic strength. The results showed that pH had a greater effect on the precipitation of peptides than salt. However, neither pI nor hydrophobicity alone could have explained the salting-out behavior; probably a combination of the two properties is responsible.