We have detected, solubilized, and purified to near-homogeneity a membrane-bound acid protease from rabbit reticulocytes. Chemical, physical, immunological, and catalytic characterization demonstrate that the enzyme is cathepsin D. With cytochrome b5 as substrate, the enzyme shows a surprisingly high pH optimum and is stimulated by ATP and DPG. Possible roles for the protease include protein processing of microsomal enzymes, degradation of subcellular organelles, and destruction of excess hemoglobin chains. The possible role of cathepsin D in protein processing of microsomal enzymes will be best assessed by the molecular biological approaches described in the following two presentations.