Guinea pig serum was chromatographed on a column of protein A-Sepharose equilibrated in citrate-phosphate buffer at pH 7.3. The bulk of serum proteins eluted in the starting buffer. Two peaks of protein A-bound serum proteins were eluted by a decreasing pH gradient, a smaller peak centered at pH 4.7 and a larger peak centered at pH 4.3. IgG contained in the peak eluted at pH 4.7 had fast gamma immunoelectrophoretic mobility and IgG in the peak eluted at pH 4.3 had slow gamma mobility. Antiserum to guinea pig IgG, when absorbed with the pH 4.7 peak, reacted only with the pH 4.3 peak. Antiserum to guinea pig IgG1, when absorbed with the pH 4.3 peak, reacted only with the pH 4.7 peak. The IgG in the pH 4.7 peak had the immunoelectrophoretic and antigenic characteristics of IgG1 and the IgG in the pH 4.3 peak had the characteristics of IgG2. The two subclasses were efficiently separated by pH-dependent affinity chromatography on protein A-Sepharose. The IgG1 in the pH 4.7 peak was contaminated with 10.8% IgG2, and the IgG2 in the pH 4.3 peak was contaminated with 2.7% IgG1.