A simple, two-step procedure to purify the immunoglobulin G (IgG) fraction from mammalian sera and ascites fluid is described. In the first step, albumin and other non-IgG proteins are precipitated with caprylic acid (octanoic acid). In the second, the IgG fraction is precipitated with ammonium sulfate. Factors influencing the precipitation of serum proteins by caprylic acid are described, as are procedural modifications to purify the IgG fraction from sera with a high lipid content. The procedure can be used to purify the IgG fraction of serum from rabbit, sheep, goat, horse, rat and mouse, as well as monoclonal antibodies from mouse ascites fluid. Greater than 80% of the IgG in rabbit serum could be isolated by this procedure, with a purity equal to rabbit IgG purified by anion-exchange chromatography. In addition to its simplicity and low cost, the procedure described offers several advantages over other methods to purify IgG.