The filarial nematode product ES-62 contains phosphorylcholine (PC) covalently attached to N-linked glycans. ES-62 induced high levels of immunoglobulin G1 (IgG1) antibodies, but no IgG2a, to non-PC epitopes of the molecule following subcutaneous injection into BALB/c mice. Conversely, mice given ES-62 lacking PC demonstrated significant production of both IgG subclasses. Thus, PC appears to block production of IgG2a antibodies to other epitopes on the parasite molecule. A role for interleukin-10 (IL-10) in this effect was shown by the ability of IL-10(-/-) mice to make an IgG2a antibody response to non-PC epitopes of ES-62.