For the characterization of null mutants identified in Drosophila populations, several Drosophila enzymes including alcohol dehydrogenase, cytoplasmic malate dehydrogenase, alpha-glycerol phosphate dehydrogenase, and phosphoglucose isomerase were co-purified to homogeneity using an 8-(6-aminohexyl)-amino-ATP-Sepharose affinity column followed by DEAE-Sepharose column chromatography. Mitochondrial malate dehydrogenase was purified by the use of CM-Sepharose and the same ATP affinity column. Alcohol dehydrogenase and alpha-glycerol phosphate dehydrogenase were mapped on a two-dimensional gel. Antiserum was raised in rabbits against these Drosophila enzymes. The presence of cross-reacting material in null mutants was characterized by double immunodiffusion, immunoelectrophoresis, and two-dimensional gel electrophoresis. By immunological techniques, two natural null variants of malic enzyme and one of phosphoglucose isomerase were shown to be negative to cross-reacting material. Two low-dose-rate gamma-radiation-induced null mutants of cytoplasmic malate dehydrogenase were shown to be positive to cross-reacting material. Two-dimensional gel analyses enabled the characterization of three natural null variants of alpha-glycerol phosphate dehydrogenase. The viability of some null mutants with homozygous null or null/deficiency genotypes is discussed in terms of the in vivo metabolic roles of the related enzymes.