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A partial sequence for α-lactalbumin, one of the two proteins of lactose synthetase, has been established. This sequence includes the exact position of all but 15 of the 123 residues in the molecule, and two of the four disulfide bonds. This partial structure shows that α-lactalbumin has a close structural similarity to hens egg white lysozyme. When the sequences of the two proteins are aligned, 40 residues in α-lactalbumin are identical with corresponding residues in lysozyme. An additional 27 residues at corresponding positions are chemically similar. From these observations it is concluded that the genes for α-lactalbumin and egg white lysozyme are derived from a common ancestor. It is proposed that an ancestral gene which controlled the sequence of a lysozyme-like enzyme duplicated, and the duplicate genes evolved independently giving rise to the genes for α-lactalbumin and the lysozymes.