The detection and partial characterization of the major histocompatibility antigens encoded by the three H-2 loci, H-2K, H-2D, and H-2L were performed by analyzing immunprecipitates from detergent-solubilized radiolabeled cells on two-dimensional gels. Each locus was found to code for a population of proteins heterogeneous in both charge and molecular weight. Only part of the heterogeneity was shown to be due to sialylation as demonstrated by shifts in mobility after neuraminidase treatment of antigen preparation. The 2-D gel analysis of the H-2K, H-2D, and H-2L antigens of the d haplotype shows that each constitutes a distinct series of products.