A 5' cDNA clone coding for human C4b-binding protein (C4bp) was isolated, characterised and sequenced to complete the cDNA sequence coding for amino-acid residues 1-32 thus confirming the protein sequence data of Chung et al. (1985c). The sequence extended to allow derivation of the putative leader sequence which was 32 residues in length and showed a high degree of hydrophobicity typical of other documented leader sequences. This clone together with others isolated by Chung et al. (1985a) were used to prepare cDNA probes which were subsequently used to hybridise genomic clones. The data from the blots suggested that the C4bp gene was up to 30 kb in size and thus consisted of large proportions of intron sequence (mRNA ≃ 2.5 kb). A commonly occurring restriction fragment length polymorphism was detected using the BglII restriction enzyme. Analysis of the genomic clones has shown that, with one possible exception, each internal protein repeat is encoded by a discrete exon. Use of the cDNA probes to hybridise RNA transfer blots, on which the RNA originated from various tissues and cell-lines, suggested that the liver is the major source of C4bp mRNA. Cross hybridisation was detected between the human C4bp cDNA probes and genomic DNA isolated from various species on transfer blots suggesting that genomic sequence homologous to that coding for C4bp has been conserved during evolution. Results from pulse field gel electrophoresis blot analysis suggests that although C4bp and the alternative pathway regulatory protein, Factor H, are genetically linked the physical distance separating them may be as great as 3 Mbp.