The movements of mouse MHC-encoded class II (H-2E) and class I (H-2K), transferrin receptor and surface Ig molecules of B lymphocytes were studied using radiolabeled mAb and electron microscopy. A total of 10 to 20% of antibodies specific for H-2E molecules were gradually internalized with a t 1/2 of 15 min, reaching a plateau after 30 min at 37 degrees C. Equivalent results were obtained either with the whole antibody or Fab' fragments, suggesting that the internalization of class II molecules was spontaneous. Similar results were obtained with antibodies specific for the transferrin receptor, of which 50% were internalized with t 1/2 of 5 min, reaching a plateau after 30 min. In contrast to antibodies specific for H-2E molecules and the transferrin receptor, antibodies specific for H-2K were not internalized. Reappearance of internalized H-2E-specific antibodies at the cell surface was observed at 37 degrees C. When compared to antibodies specific for surface Ig, degradation of antibodies specific for H-2E molecules was limited even after 5 h incubation. Neither ammonium chloride nor cycloheximide inhibited internalization and recycling. Electron microscopy showed that internalization of H-2E molecules occurred via coated pits/coated vesicles. These results indicate that class II molecules are spontaneously internalized and recycled by B lymphocytes.