Degradation of the amyloid precursor protein (APP) by lysosomes has been proposed to be the mechanism for generation of the beta/A4 polypeptide which is the major constituent of amyloid plaques. In this report, we use inhibitors to elucidate the steps involved in the lysosomal degradation of APP in PC12 cells. Monensin treatment significantly elevated the level of immature APP. Reducing the temperature to 17 degrees C, adding cytochalasin B and colchicine, or exchanging K+ for Na+ resulted in a substantial accumulation of both mature and immature APP isoforms. The inhibitor of autophagy, 3-methyladenine, had no effect on the level of APP isoforms. These results suggest that changes in ionic balance, membrane fluidity or vesicle fusion may affect APP processing.