The weak bases chloroquine, primaquine, NH4Cl and the ionophore monensin exert similar but not identical effects on sorting, transport and processing of cathepsin D in several human cell lines (fibroblasts, HepG2 cells, U937, monocytes). The drugs inhibit the segregation of newly synthesized cathepsin D from the secretory route. The kinetics of transport of nonsegregated cathepsin D precursor along the secretory route is retarded resulting in a delayed hypersecretion. Higher concentrations of the drugs can arrest the intracellular transport completely. The extent of inhibition of segregation varies among the different human cell types tested. Thus, in fibroblasts the secretion can be stimulated to exceed 80%, while in U937 cells the secretion cannot be enhanced above 50% although both cell types have the same basal rate of secretion (approximately 10% of the synthesized cathepsin D). We suggest that pH-independent sorting mechanisms contribute to the targeting of cathepsin D in U937 cells. Processing of the cathepsin D remaining in cells is characteristically changed depending on the drug. The proteolytic processing is strongly inhibited by chloroquine and is rather insensitive to monensin. Unlike the other drugs, monensin blocks the formation of complex oligosaccharides in cathepsin D and allows for extensive secretion solely of molecules that are sensitive to endo H.