The addition of hyaluronan to the culture medium of explant cultures of articular cartilage was shown to suppress the synthesis of hyaluronan and aggrecan, the major proteoglycan present in cartilage, and resulted in a greater proportion of the newly synthesized aggrecan and hyaluronan appearing in the culture medium. This effect of exogenous hyaluronan on aggrecan and hyaluronan synthesis was concentration-dependent and reversible on removal of the glycosaminoglycan from the culture medium. The addition of tetra- and hexasaccharides derived from Streptomyces sp. hyaluronidase digestion of hyaluronan to explant cultures of articular cartilage did not change the rate of synthesis of aggrecan or hyaluronan or their ultimate distribution between tissue and medium. However, the addition of tetra- and hexasaccharides of hyaluronan resulted in a decrease in the rate of loss of hyaluronan from the tissue but not that of aggrecan, which remained the same as in control cultures. This suppression of the rate of loss of hyaluronan was eliminated on removal of the hyaluronan oligosaccharides from the culture medium. Analysis of the hydrodynamic size of the newly synthesized hyaluronan indicated that the presence of hyaluronan tetra- and hexasaccharides brought about an accumulation of hyaluronan of intermediate molecular mass. Since no radiolabeled hyaluronan was detected in the culture medium, it was concluded that the tetra- and hexasaccharides inhibited the internalization and intracellular catabolism of hyaluronan by the cartilage explant cultures. Regardless of whether hyaluronan or tetra- and hexasaccharides of hyaluronan were added to the culture medium, newly synthesized hyaluronan underwent depolymerization at a rate consistent with a mechanism involving oxygen-derived radicals.