The collagens are a family of structural proteins which function as an extracellular framework in eukaryotic organisms. They are characterised by a unique protein conformation which consists of three polypeptide chains in a triple helix. At least twelve collagen types encoded by at least twenty non-allelic genes have been identified in vertebrates. There is considerable evidence that each member of the family is represented by a single copy gene. These genes constitute a multi-gene family with a common evolutionary origin. Some of the genes are known to be clustered on certain human chromosomes. The collagens have an extremely important role in development. Alterations in collagen genes can result in a heterogenous group of heritable diseases of connective tissue. There is accumulating evidence that similar phenotypes are due to similar mutations or location of mutations. DNA sequencing studies of cDNA clones of the human type III procollagen gene revealed single base polymorphisms, and one amino acid polymorphism, by comparison with published data. These may be used to generate haplotypes at this locus and increase the polymorphic content for genetic analysis. An attempt to isolate the human type III procollagen gene by cosmid cloning failed. Due to the technical difficulties of performing control experiments, the reasons for this failure could not be identified with certainty. It is possible that this gene is not amenable to cloning using the commonly used cloning reagents. A study of the human type I procollagen gene, COL1A1, revealed the presence of a monomorphic repeat sequence in an intron. This repeat sequence was used to identify a multi-allelic locus in the terminal region of the short arm of chromosome 19. The usefulness of this locus in linkage studies to disorders that map close to this region remains to be analysed.