The effects of brefeldin A, monensin, and the weak base NH₄C1 on the biosynthesis and processing of cathepsin G and neutrophil elastase of myeloid cells were investigated. Monoblast‐like U‐937 cells were biosynthetically labeled with [³⁵S]methionine, followed by subcellular fractionation, immunoprecipitation, and analysis by sodium dodecyl sulfate‐polyacrylamide gel electrophore‐sis and fluorography. Brefeldin A inhibited proteolytic processing, intracellular transport, and secretion. The effects were reversible inasmuch as removal of brefeldin A resulted in a normal pattern of processing and transfer to high‐density organelles, corresponding to lysosomes, and restitution of constitutive secretion of precursor forms. Both cathepsin G and neutrophil elastase acquired resistance to endoglycosidase‐H, suggesting that conversion to complex oligosaccharide side chains also occurs in the presence of brefeldin A. Monensin and NH₄Cl inhibited final proteolytic processing, indicating either that acidification is necessary for directing cathepsin G and neutrophil elastase to lysosomal‐like organelles or that the protease(s) responsible for processing requires an acid pH. We conclude that pH‐dependent proteolytic processing of cathepsin G and neutrophil elastase occurs in post‐Golgi structures and that transfer to lysosomes or an immediately prelysosomal compartment is mandatory for complete processing. J. Leukoc. Biol. 55: 50–57; 1994.