A model for the mechanism of protein precipitation by caprylic acid (CA) is developed on the basis of quantitative assays of precipitation with bovine serum albumin (BSA) and CA at different concentrations. It was found that the effect of CA is due to direct interaction with the precipitating protein. Maximum precipitation was achieved when the mass ratio of CA–BSA was close to 1, equivalent to about 450 CA molecules per molecule of BSA. This value was confirmed by optimizing the CA purification of immunoglobulins from equine blood plasma. With a sample diluted 1:1, it was found that CA at a final concentration of 3.5% is optimal to obtain immunoglobulins essentially free of albumin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It is proposed that CA binds to specific sites of the protein, thereby inducing partial unfolding of the protein, which exposes additional binding sites. More CA molecules incorporate into all sites in the form of mixed micelles. Thus, the interfacial protein surface becomes highly hydrophobic and increases protein–protein attraction, causing association and precipitation of the macromolecular complexes.