Abstract The influence of self-association on the precipitation of proteins by poly(ethylene glycol) (PEG) was investigated using several model self-associating proteins. These include α-chymotrypsin and chymotrypsinogen, β-lactoglobulin A and glutamate dehydrogenase. With few exceptions, conditions which promote self-association also enhance precipitation by PEG, while conditions which inhibit self-association also inhibit precipitation by PEG. Chymotrypsin and chymotrypsinogen self-associate at pH 8.5 and low ionic strength and, under these conditions, they were precipitated by moderate concentrations of PEG (10–20%, w v ). Low concentrations of salts (<0.1 m) reversed the self-association and prevented precipitation up to 25% PEG. By contrast, salt did not prevent the precipitation of covalently cross-linked oligomers of chymotrypsinogen. In acid (pH 4.5), the effects of salts on both self-association and precipitation of these proteins were opposite to the effects at pH 8.5. Different salts were generally found to have similar effects, but SO 4 2− was particularly effective in enhancing precipitation in acid. At pH 3 and 8, β-lactoglobulin A is primarily a monomer and was not precipitated by 25% PEG at these pH values. Maximum precipitation occurred at pH 4.7, which is the reported optimum for self-association. Glutamate dehydrogenase associates to polymers at neutral pH and was precipitated by 15% PEG. The combined presence of NADH and GTP (1 m m) prevented polymer formation and abolished precipitation. Toluene further enhanced polymerization in the absence of cofactors but did not enhance precipitation. However, PEG was found to strongly inhibit the polymerization induced by toluene. The results of these studies suggest that it may be possible to selectively enhance or inhibit the PEG precipitation of a self-associating protein through manipulation of its oligomeric state.