The initiation and propagation of the immune responses is dependent on the ability of antigen-presenting cells (APC) to convert proteins into peptides, to load them intracellularly onto major histocompatibility complex (MHC), and then to deliver the peptide-MHC complexes to the plasma membrane in order to stimulate T cells. It is becoming increasingly clear that most of the intracellular compartments that contain MHC class II products in APC simply represent the conventional endosomes and lysosomes that are expressed in all cell types (1 ). However, data from cell-fractionation studies, predominantly those using electrophoresis techniques, show that in professional APC, a class of class II-containing endosomes that is specialized for antigen processing may exist. Strong support for this possibility comes from the observation that such specialized structures, designated CIIV, are particularly abundant in mature dendritic cells (DC; ref 2 ).