Immunological characterization of serine-glyoxylate aminotransferase and hydroxypyruvate reductase, key enzymes for the assimilation of one-carbon compounds in methylotrophs, was performed using antibodies raised against these enzymes purified from Hyphomicrobium methylovorum GM2. Immunodiffusion studies indicated that serine-glyoxylate aminotransferase and hydroxypyruvate reductase of all seven Hyphomicrobium strains tested were immunochemically similar. In immunotitration experiments and Western blot analyses of both enzymes in the genera Hyphomicrobium and Methylobacterium, the serine-glyoxylate aminotransferase of the genus Methylobacterium exhibited low similarity to that of the genus Hyphomicrobium. For hydroxypyruvate reductase, no immunological relationship was observed between the genera Hyphomicrobium and Methylobacterium, which was in agreement with the differences in primary structure and enzymological properties.