The primary structure of a monoclonal human IgA-immunoglobulin has been determined. Sequence studies were carried out with the isolated L-chain [1], the isolated H-chain and with BrCN-fragments of the H-chain. Tryptic and chymotryptic peptides were prepared from the totally reduced and alkylated alpha-chain or from BrCN-fragments. Sequence work has been done with tryptic as well as chymotryptic peptides. The variable part comprised positions 1-119, the constant part residues 120-472. According to its homology with other variable parts alpha-chain Tro. clearly belongs to subgroup III of the H-chains. The constant part is composed of 3 homology regions (C1-C3) which originated from a common ancestor by repeated gene duplications early in evolution. Each homology region corresponds in its length and its sequence to the C-region of the L-chains. The hinge-region, which connects the C1- and the C2-region, originated from the C-terminal end of the C1-region by a twofold partial gene duplication comprising 8 amino acids. The C3-homology region is termined by an additional C-terminal piece of 18 residues. The alpha-chain 17 cysteine residues, 8 of which form the usual intrapeptidal loop S-S-bridges, and one the connection between the L- and H-chain. Two additional cysteine residues are located in the C1-region, and 5 more in the C2-region, forming intra- and inter-peptidal S-S-bonds.