An antiserum against the purified glucose-6-phosphate dehydrogenase (EC 1.1.1.49) (G-6-P-DH) of L. mesenteroides NCDO 768 was prepared. Cross-reactions were observed with extracts of other strains of Leuconostoc, comprising named strains of L. lactis, L. mesenteroides, L. dextranicum, L. cremoris and L. paramesenteroides. Cross-reactions were not obtained with L. oenos. The groups of identical specificity and their order of decreasing similarity toward the homologous enzyme were determined and the results are consistent with those obtained previously with another G-6-P-DH antiserum from L. lactis NCDO 546. Except for L. oenos, many strains of Leuconostoc comprising multiple (3 or more) species might be considered as phenotypically diverse but genetically rather uniform in their line of descent from a common ancestor.