A partial cDNA clone for the A-chain of human complement subcomponent C1q was isolated from a monocyte library. Use of the A-chain cDNA clone, and a previously characterized B-chain cDNA clone [Reid (1985) Biochem. J. 231, 729-735] allowed the isolation of overlapping cosmid clones that were shown to contain the genes encoding the A-, B- and C-chains of human C1q. The three genes were found to be aligned, 5'----3', in the same orientation, in the order A-C-B on a 24 kb stretch of DNA on chromosome 1p. The A-, B- and C-chain genes are approx. 2.5, 2.6 and 3.2 kb long respectively, and each contains one intron, located within a codon for a glycine residue found half-way along the collagen-like region present in each chain. These glycine residues are located just before the point where the triple-helical portions of the C1q molecule appear to bend when viewed in the electron microscope. Southern-blot analyses indicated that there is only one gene per chain, and preliminary examination of genomic DNA from several C1q-deficient patients showed no evidence for major deletions or insertions within the A-, B- or C-chain genes. The DNA sequence of the coding region of the C-chain gene allows the completion of the entire derived amino acid sequence for the human C1q molecule. The globular, C-terminal, regions of the chains of C1q show a strong similarity in amino acid sequence to the non-collagen-like, C-terminal, regions of the type VIII and type X collagens, indicating structural and evolutionary relationships between these three molecules.