The IgG subclass distribution of rat antibodies to 13 different antigens was measured. Antibodies to protein and hapten-protein conjugates were predominantly IgG2a. Antigens labeled thymus-independent type 1, based upon responses in mice, stimulated both IgG2b and IgG2c antibodies, but little IgG2a. Polysaccharide and hapten-polysaccharide antigens (thymus-independent type 2) as well as phosphocholine-keyhole limpet hemocyanin, stimulated predominantly IgG2c antibodies. A division of antigens into essentially the same categories has been made on the basis of subclass restriction in mice. Antigens that stimulate IgG2c in rats stimulate IgG3 in mice. Thus, by comparing subclass preference with a variety of antigens, functional analogues among subclasses in different species can be identified.