The Swarm rat chondrosarcoma cell line, RCS-LTC, deposits an extracellular matrix that contains the typical type II, IX, and XI collagen phenotype of hyaline cartilage, but the fibrils appear abnormally thin. By N-terminal sequence analysis, the type II collagen from the matrix was shown to have retained its N-propeptides with no evidence of normal processing to type II collagen. Amplification and sequencing of cDNA prepared from the pro alpha1(II) mRNA of these cells showed a normal N-propeptide cleavage site. Furthermore, the type II N-procollagen could be processed to type II collagen by incubation with culture medium from normal chondrocytes. The findings indicate that the RCS-LTC cell line fails to express an active type II procollagen N-proteinase and, therefore, offers a useful culture system in which to study the role of N-propeptide removal in fibrillogenesis.