Carbohydrate and protein antigens have been shown to elicit the bulk of their antibodies in mutually exclusive IgG subclasses in humans, mice, rats and horses1-4. In the mouse, anti-protein antibodies are primarily of the IgG1 subclass, and anti-carbohydrate antibodies are primarily of the IgG3 subclass1,2. We have now demonstrated that mouse IgG3 antibodies to the phosphocholine (PC) determinant of pneumococcal C-carbohydrate5,6 and totype 3 pneumococcal polysaccharide are highly protective against experimental type 3pneumococcal infection. This is the first demonstration that antibodies of the IgG3 subclass can protect against bacterial infection. © © 1981 Nature Publishing Group.