We report here the identification of a common immunological determinant in Escherichia coli and Bombyx mori (silkworm) alanine tRNA synthetases. The E. coli protein is a tetramer of identical Mr = 95,000 chains, and the silkworm enzyme is a monomer of Mr = 115,000. Antibodies against the silkworm enzyme react with E. coli Ala-tRNA synthetase. Analysis of 10 fragments of the E. coli enzyme has mapped the cross-reacting epitope to between amino acids 350 and 385. This is within the part of the enzyme which is essential for alanyladenylate synthesis. The anti-B. mori Ala-tRNA synthetase antibodies which cross-react with the E. coli enzyme were affinity-purified. They react specifically with the catalytic domain of the silkworm enzyme and not with the remaining dispensable segment of 500 amino acids. The results support the concept that the core catalytic structural elements, and not the dispensable portions, are the most related among the synthetases.