Tibial dyschondroplasia (TD) is an abnormality of the growth plate of growing chicks characterized by the presence of a mass of nonvascularized, nonmineralized cartilage. To determine if the morphologic changes observed in TD lesions are accompanied by biochemical abnormalities of the cartilage matrix, we performed quantitative and qualitative analyses of collagen in affected cartilage, placing special emphasis on the study of type X collagen, a recently described molecular species that may play a role in endochondral ossification. Collagen from TD lesions of 25-day-old chicks and from growth plate cartilage of normal age-matched chickens and from hypertrophic cones of 1-day-old chicks was sequentially extracted, purified and characterized. No differences were noted in total collagen content and extractibility between these tissues. Determination of the relative amounts of the various collagens extracted showed that type X collagen in the TD lesion was markedly reduced since it was less than half of that found in pure hypertrophic cartilage. These results are consistent with previous morphologic and biochemical studies which suggest a failure of the chondrocytes to fully achieve maturation and hypertrophy. No differences were demonstrated in the CNBr peptide patterns or amino acid composition of type X collagen from normal or TD and hypertrophic cartilages, suggesting that the low content of type X collagen in TD lesion was due to alterations in type X collagen metabolism rather than to structural abnormalities in these molecules.