Bovine aggrecan was digested with bovine cathepsin D at pH 5.2 under conditions of partial digestion and the resulting aggrecan core protein fragments were separated by electrophoresis on gradient polyacrylamide gels. The fragments were characterized by their reactivity to specific antibodies and by N-terminal amino acid sequencing. It was also demonstrated that cathepsin D cleaved bovine aggrecan at five sites within the core protein, between residues Phe³⁴²–Phe³⁴³ in the interglobular domain, Leu¹⁴⁶²–Val¹⁴⁶³ between the chondroitin sulfate attachment regions 1 and 2 and Leu¹⁶⁵⁴–Val¹⁶⁵⁵, Phe¹⁷⁵⁴–Val¹⁷⁵⁵ and Leu¹⁸⁵⁴–Ile¹⁸⁵⁵ that are located within the chondroitin sulfate attachment region 2 of the core protein. The time course of digestion showed that there was a continued degradation of aggrecan and there was no preferential cleavage of the core protein at any one site. It was shown that cathepsin D digested aggrecan over the pH range 5.2–6.5 resulting in the same products. When bovine cartilage was maintained in explant culture at pH 5.2 there was a rapid loss of both radiolabeled and chemical pools of sulfated glycosaminoglycans into the culture medium and this loss was inhibited by the inclusion in the medium of the aspartic proteinase inhibitor, pepstatin A. The aggrecan core protein fragments appearing in the medium of cultures maintained at pH 5.2 were characterized and it was shown that the fragments had N-terminal sequences starting at Phe³⁴³, Ile¹⁸⁵⁵, and Val¹⁷⁵⁵ or Val¹⁴⁶³. This work demonstrates that cathepsin D present within the extracellular matrix of articular cartilage has the potential to contribute to the proteolytic processing of the core protein of aggrecan in this tissue.