In B lymphocytes, the processing of exogenous proteins and the subsequent binding of antigenic peptides to class II molecules encoded by the major histocompatibility complex (MHC) occurs most likely within endocytic compartments. To examine the endocytic transport of MHC class II molecules, we used (i) surface iodination followed by internalization, pronase treatment and immunoprecipitation, (ii) in situ iodination of endosomal compartments, and (iii) confocal microscopy to visualize the fate of fluorescence coupled Fab fragments. In murine I-Ak, I-Ek positive B lymphoma cells, cell surface MHC class II molecules are partially protected from pronase digestion after 15 min at 37 degrees C and recycled back to the cell surface within the next 30 min. The fluorescence coupled Fab fragments are delivered to juxtanuclear endocytic compartments in 15 min. In contrast to the murine B cells, L fibroblasts transfected with either I-A alpha beta k or I-E alpha k beta k,d fail to internalize their surface class II molecules. A fraction of class II molecules, however, is still present in endosomal compartments as detected after in situ iodination in L fibroblasts. We conclude that the recipient L fibroblasts lack one or several factors needed for the transport of MHC class II molecules from the cell surface to the endosomes. We suggest that in murine B lymphoma cells, antigenic peptides can gain access to a pool of recycling class II molecules whereas in L cells they meet newly synthesized class II molecules targeted to the endosomal compartments.