Lipid kinases and their phosphorylated products are important regulators of many cellular processes, including intracellular membrane traffic. The best example of this is provided by the class III phosphoinositide 3-kinase (PI-3K), Vps34p, which is required for correct targeting of newly synthesized carboxypeptidase Y to the yeast vacuole. A probable mammalian Vps34p orthologue has been previously identified, but its function in the trafficking of lysosomal enzymes has not been resolved. To investigate the possible role(s) of mammalian Vps34p in protein targeting to lysosomes, we have cloned the rat orthologue and overexpressed a kinase-deficient mutant in HeLa cells. Expression of the mutant protein inhibited both maturation of procathepsin D and basal secretion of the precursor. In contrast wortmannin, which also inhibited maturation, caused hypersecretion of the precursor. We propose that mammalian Vps34p plays a direct role in targeting lysosomal enzyme precursors to the endocytic pathway in an analogous fashion to its role in the fusion of early endocytic vesicles with endosomes. We further suggest that inhibition of a wortmannin-sensitive enzyme, other than mammalian Vps34p, is responsible for the failure to recycle unoccupied mannose 6-phosphate receptors to the trans-Golgi network, and consequent hypersecretion of lysosomal enzyme precursors observed in the presence of this drug.