Pig ficolins and a number of other proteins contain sequences that are homologous to the C-terminal halves of fibrinogen beta- and gamma-chains. To clone the cDNA for human ficolin, two degenerate oligonucleotide primers were synthesized, based on two stretches of protein sequence that were highly conserved among those proteins, and used for PCR with cDNA from a human uterus lambda gt11 library as a template. A PCR product with a predicted size of 300 bp was obtained and this was used to screen a uterus cDNA library. Of the positive clones isolated, two (U1 and U2), containing inserts of 1.7 and 1.1 kb respectively, were found to encode human ficolin. The cDNA-derived amino acid sequence of human ficolin has approx. 75% identity with, and a similar domain organization to, the two pig ficolin sequences, which are characterized by the presence of a leader peptide, a short N-terminal segment followed by a collagen-like region and then by a C-terminal fibrinogen-like domain. The 1.1 kb insert of clone U2 was used in Northern-blot analysis, and a very strong signal for a 1.4 kb mRNA species was detected in mRNA from human peripheral blood leucocytes. This showed that, despite the initial characterization of pig ficolin as a putative receptor on uterine cells for transforming growth factor beta 1, blood leucocytes are probably the major site of human ficolin synthesis. Much weaker signals of the same size were also detected in spleen, lung and thymus and may be due to the presence of tissue macrophages or trapped blood in these tissues. An mRNA species of approx. 1.3 kb in human liver also weakly hybridized to the U2 probe, indicating the presence of a sequence that was distinct from, but related to, ficolin. The gene for human ficolin has been mapped to chromosome 9.