C4 has been isolated from human plasma by a three-step procedure and its component chains have been separated. Partial amino acid sequence analysis of these chains reveals considerable polymorphic variability. Chains isolated from individual donors have been examined to define this polymorphism at the primary structural level. The structure of the γ-chain has been more intensively investigated using fragmentation with CNBr, and the resulting peptides have been partially sequenced. The longer CNBr fragments of the γ-chain have been further fragmented with trypsin after citraconylation. Sequence data concerning these peptides will be presented and discussed.