Murine spleen cell surface IgD is found in two forms upon SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Both IgDI and IgDII are present on the surface of intact cells and appear to be native structures. Neither form could be accounted for by proteolytic degradation or disulfide bond rearrangement. IgDII has an apparent m.w. of 96,000, suggesting an HL structure. IgDI has an apparent m.w. of 150,000, approximately 33,000 daltons lower than that expected for an H2L2 structure. Significant variation in the relative amounts of IgDI and IgDII was found when mice of different strains were examined.