Ther serum concentration of normal adult goat total IgG was established to be 19.97 +/- 1.55 mg/ml, the IgG1 10.92 +/- 0.84 mg/ml and IgG2 9.07 +/- 0.78 mg/ml. No significant variations were found to be associated with the seasons of the year but changes in concentration, especially in serum IgG1 occur ante- and post-partum. In goat colostrum, the IgG concentration is about 2.4-2.8 times greater than in serum and the IgG1 subclass accounts for 95-98 per cent. During the immune response the IgG1 rises sharply whereas variations in IgG2 concentration are less evident. Both IgG subclasses are active in haemagglutination, although the IgG1 is 22-52 times more efficient. As in all ruminants, only IgG1 fixes complement in the classical test. Differences exist between IgG subclasses in their ability to induce PCA reactions. IgG2 subclass is active only in homologous species whereas the IgGl in heterologous species. Cytophilic activity is associated with IgG2 subclass.