THE peptide/class II major histocompatibility complex (MHC) complexes recognized by CD4(+) T cells have been characterized at the structural(1) and biochemical(2-4) levels and studies on the transport and maturation of class II MHC indicate that specialized sites may be involved in peptide acquisition(5-11). Here we report the characterization of the compartments involved in antigen processing and class II MHC loading relative to distinct functional domains of the endocytic pathway in antigen-specific human B lymphocytes(12-14). Peroxidase-mediated crosslinking analysis(15) in intact cells demonstrates that peptide loading of class II MHC takes place in a compartment accessible to membrane immunoglobulin but not to transferrin receptors, although processing may be initiated within the latter domain. The density of membrane vesicles carrying newly assembled class II MHC complexes was distinct from early and late endosomes and dense lysosomes. Endocytosed antigen-gold complexes enter a class II MHC-rich compartment morphologically very similar to that described previously(9) and within the time frame of biochemically detectable peptide loading.