A study of the interaction of proteins [soybean trypsin inhibitor (0.3-1 mg/mL)] with dilute solutions of synthetic polyampholytes (0.05-0.4 mg/mL) composed of acrylic (or methacrylic) acid, (dimethylamino)ethyl methacrylate, and methyl methacrylate was undertaken experimentally using turbidimetric titration. The polyampholytes had a number-average molecular weight of 4000 (block polyampholytes) or 70 000 (random polyampholytes) and a polydispersity index of only 1.3. Alone, the polyampholytes showed a tendency to self-aggregate around their isoelectric point. Therefore, polyampholyte-polyampholyte interactions (self-aggregation) and polyampholyte-protein complexation behavior were studied as a function of pH (3-9), ionic strength (0-0.75 M) and polymer dosage (50-400 mg of polymer/g of protein). Large increases in turbidity (>500%) were observed for protein-polyampholyte mixtures (compared to polyampholyte alone) in the case of both block and random copolymers. The observed turbidity increases could be due to protein-polymer interactions or to enhanced precipitation of polymer in the presence of protein. To differentiate between these alternatives, turbidimetry experiments were carried out at different dosages by (a) keeping the protein concentration constant and varying the polymer concentration and Co) keeping the polymer concentration constant and varying the protein concentration in small steps; they demonstrated that the turbidity increases were indeed due to protein-polymer interactions. These experiments offer a general methodology to distinguish between intermolecular protein-polymer and intermolecular polymer-polymer interactions.