Two acid alpha-glucosidase components of different molecular sizes (80 and 71 kDa) were separated from human placenta by DEAE-cellulose chromatography. Their catalytic properties were similar, and they showed almost the same molecular structure with regard to immunological properties and carboxy-terminal sequences, although the amino acid composition, the total hexose content, and the circular dichroism spectra were different. The pulse-chase labeling acid alpha-glucosidase with [3H]leucine revealed a processing pathway from a 110 kDa precursor to a 95 kDa intermediate form, then finally to 80 and 71 kDa mature forms. However, the sequence of appearance was different between the two mature enzymes. The 80 kDa component appeared first after chase for 5 h, and then the 71 kDa component followed. Their amounts became equal at 2 to 4 days. When ammonium chloride or leupeptin was added to the culture medium after chase for 5 h, the 71 kDa component failed to appear and the 80 kDa component was not converted to 71 kDa. It is concluded that probably only a part of the 80 kDa component is processed to form the 71 kDa component, although another possibility that cannot be excluded is that these two components are converted independently from the common intermediate 95 kDa protein.