Limited proteolysis by pepsin at pH 4.5 of non-specific pig IgG and of two types of pig anti-dinitrophenyl antibodies revealed striking differences in susceptibility to proteolytic cleavage. The digests were resolved on a column of Sephacryl S-200 and the elution profiles analysed using a computer programme. The individual fragments were characterized by immunoelectrophoresis, SDS-polyacrylamide gel electrophoresis and molecular mass determination. Whereas the non-precipitating antibodies and the non-specific IgG yielded F(ab')2 and pFc' fragments in reasonable quantities, the precipitating antibody proved to be stable even at the 70th h of digestion. The precipitating antibody became susceptible to peptic cleavage only when the pH was lowered to 4.0. It was concluded that the precipitating antibody represents a subclass of pig IgG with an anomalous resistance to proteolysis at acidic pH and that this subclass constitutes only a few per cent, at most, IgG of the healthy, non-immunized animals.