As an approach to elucidate the role of collagen XIV, which is still unclear, molecules exhibiting affinity for this collagen have been sought in connective tissue. Extracts from fetal bovine tendon were resolved by gel electrophoresis and electrophoretically transferred to nitrocellulose. The blot was overlaid with native collagen XIV and the collagen XIV-binding molecules revealed by immunodecoration with a monoclonal antitype XIV collagen antibody. This experimental approach allowed us to reveal in tendon extracts a diffuse band, with an apparent molecular mass of approximately 100 kDa, that binds collagen XIV. This molecule was also found associated with the fractions containing partially purified type XIV collagen. This 100-kDa molecule was sensitive to chondroitinase ABC and, after chondroitinase digestion, yielded a core protein of about 48 kDa. N-terminal sequence analysis of the proteoglycan after blotting allowed us to identify it as decorin. By solid phase assays we have studied this newly described association between decorin and type XIV collagen and shown that it is a saturable process. In addition, preliminary determination of the domains of the two molecules involved in the association has been performed. The possible role of these interactions is discussed.