We have previously identified and purified transforming growth factor-beta 1 (TGF-beta 1)-binding proteins from porcine uterus membranes (Ichijo, H., Rönnstrand, L., Miyagawa, K., Ohashi, H., Heldin, C.-H., and Miyazono, K. (1991) J. Biol. Chem. 266, 22459-22464). One of these TGF-beta 1-binding proteins, with a molecular weight of 40,000, was purified to homogeneity and subjected to amino acid sequence analysis. The amino acid sequences obtained were used to isolate two closely related cDNA clones from a porcine uterus cDNA library. The deduced amino acid sequences revealed that both cDNAs encoded proteins that were mainly composed of fibrinogen-like and collagen-like domains. Therefore, they were denoted ficolin-alpha and ficolin-beta. Expression of ficolin-alpha and -beta cDNA in mammalian cells revealed that ficolin forms dimers, trimers, and several higher order of oligomers, whose molecular weights fit well with those of the purified TGF-beta 1-binding proteins from porcine uterus. Moreover, immunoblotting analysis using a peptide anti-serum against ficolin indicated that the TGF-beta 1-binding proteins identified in porcine uterus are ficolin-alpha, -beta, and their oligomers or closely related molecules. However, recombinant ficolin-alpha and -beta did not bind TGF-beta 1, despite the similarities in molecular weights and immunoreactivity with the material from the natural source. It is possible that a specific posttranslational modification of ficolin or interaction with another component is needed for TGF-beta 1 binding. Analysis by Northern blotting revealed that the expression of ficolin-alpha mRNA is relatively restricted and most abundant in placenta and lung. On the other hand, ficolin-beta was mainly expressed in skeletal muscle. The in vivo functions of ficolin will be discussed.