Southern blot analysis of the B-chain genes in one of eight C1q-deficient individuals revealed an abnormal banding pattern. The defect, which was homozygous, could be localized by restriction mapping to a single Taq I site within residue 150 in the coding region of the B-chain gene. DNA sequencing across the site revealed a stop codon that would cause premature termination of the protein product. No material corresponding to the A or C chains, or a truncated B chain, could be identified by antigenic analysis of the patient's serum, indicating that a complete B chain is required for secretion of a C1q molecule.