Total tissue content and molecular mass of hyaluronic acid was determined in papain digests of human articular cartilage using a sensitive radiosorbent assay [Laurent & Tengblad (1980) Anal. Biochem. 109, 386-394]. 1) Hyaluronic acid content increased from 0.5 microgram/mg wet wt. to 2.5 micrograms/mg wet wt. between the ages of 2.5 years and 86 years. 2) Hyaluronic acid chain size decreased from Mr 2.0 x 10(6) to 3.0 x 10(5) over the same age range. 3) There was no age-related change in the size of newly-synthesized hyaluronic acid, which was of very high molecular mass, in both immature and mature cartilage. The results are consistent with an age-related decrease in proteoglycan aggregate size and suggest that modification of the hyaluronic acid chain may take place in the extracellular matrix.