The failure of cartilage mineralization in osteochondrotic cartilage may be due to an impaired proteoglycan production. The in vitro production of proteoglycans was therefore studied in the joint cartilage of growing pigs, aged 9-18 weeks, after incubation of cartilage samples with 35S-sulfate. Cartilage was obtained from different areas of the femoral condyles and samples from these areas were further divided into three layers, where the superficial layer contains articular cartilage and the basal layers consist of growth cartilage. There was no significant difference in the overall amount of 35S-proteoglycans synthesized in different areas of the condyles. However, the total production of 35S-proteoglycans per mg tissue was highest in the basal layer in all areas. This was not due to a larger number of cells; the superficial layer contained more DNA per mg tissue than the basal layer. Gel chromatography on Sepharose CL-2B of the cartilage extracts, which resulted in the separation of large proteoglycans (Kav approximately 0.4) from proteoglycans of small hydrodynamic size (Kav approximately 0.8), showed that the relative amount of large proteoglycans increased with the distance from the articular surface. Again, no difference in the relative amounts of large and small proteoglycans were found when cartilage from different areas were compared. Osteochondrotic cartilage was detected in the pigs aged 12-18 weeks. In areas where osteochondrotic cartilage were present, the total production of 35S-proteoglycans was lowered and the relative amount of large proteoglycans was less than that found in the adjoining areas devoid of osteochondrotic lesions.(ABSTRACT TRUNCATED AT 250 WORDS)