The complete primary structure of the constant part of the u-chain-disease protein, BOT, was established. It includes the whole CH2, CH3 and CH4 domains. Two amino acid changes were found, at positions 309 (Ser→Gly) and 333 (Val→ Gly) (GAL numbering). In two additional monoclonal μ chains (SCO and CO), the same positions showed an amino acid variability. From these data it may be concluded that four types of μ chains exist in the human: (1) GAL type with Ser-309 and Val-333; (2) OU type with Gly-309 and Val-333; (3) SCO type with Ser-309 and Gly-333; (4) BOT/CO type with Gly-309 and Gly-333. The meaning of this molecular polymorphism is discussed.