The nucleotide sequences coding for murine complement component C3 have been determined from a cloned genomic DNA fragment and several overlapping cloned complementary DNA fragments. The amino acid sequence of the protein was deduced. The mature beta and alpha subunits contain 642 and 993 amino acids respectively. Including a 24 amino acid signal peptide and four arginines in the beta-alpha transition region, which are probably not contained in the mature protein, the unglycosylated single chain precursor protein preproC3 would have a molecular mass of 186 484 Da and consist of 1663 amino acid residues. The C3 messenger RNA would be composed of a 56 +/- 2 nucleotide long 5' non-translated region, 4992 nucleotides of coding sequence, and a 3' non-translated region of 39 nucleotides, excluding the poly A tail. The beta chain contains only three cysteine residues, the alpha chain 24, ten of which are clustered in the carboxy terminal stretch of 175 amino acids. Two potential carbohydrate attachment sites are predicted for the alpha chain, none for the beta chain. From a comparison with human C3 cDNA sequence (of which over 80% has been determined) an extensive overall sequence homology was observed. Human and murine preproC3 would be of very similar length and share several noteworthy properties: the same order of the subunits in the precursor, the same basic residue multiplet in the beta-alpha transition region, and a glutamine residue in the thioester region. The equivalent position of the known factor I cleavage sites in human C3 alpha could be located in the murine C3 alpha chain and the size and sequence of the resulting peptide were deduced. A comparison of the amino acid sequences of murine C3 and human alpha 2-macroglobulin is given. Several areas of strong sequence homology are observed, and we conclude that the two genes must have evolved from a common ancestor.