Analysis of the knee-joint cartilage of pigs at five ages (namely foetuses from the second half of pregnancy and animals 10 weeks, 25 weeks, 3 years and 5 years old) showed that the composition approached that of adult cartilage by 25 weeks of age, the most marked differences being between foetal and 10 week-old cartilage. Protein-polysaccharides were extracted sequentially, first by brief low-speed homogenization with iso-osmotic sodium acetate, then by two extractions with 2m-CaCl(2) for 24h with gentle agitation interspersed with brief low-speed homogenization and agitation for another 24h. About half of the protein-polysaccharides were removed from foetal cartilage by the first extraction and the remainder by the second. The proportion in the first extract declined sharply with the age of the animal, but that in the first CaCl(2) extract was similar at all ages other than 10 weeks. The amount left in the residue increased approximately with the collagen content from about one-fifth at 10 weeks of age to one-third in adult and old cartilage. The proportion of medium-sized protein-polysaccharides in the extracts changed little with age after birth, but the glucosamine content increased about fivefold and the protein content almost doubled between 10 weeks and 5 years of age. Other analytical values changed little. These results cannot be explained solely by changes in the proportion of ;link-glycoprotein' in the protein-polysaccharides. Since major changes in most parameters had taken place by 25 weeks of age, the first weeks after birth may be a critical period for cartilage development in the pig.