"Malic" enzyme (EC 1.1.1.40) has been purified from rat mammary tissue and rat liver using a three stage procedure involving ammonium sulphate precipitation, affinity chromatography and ion exchange chromatography. The enzymes from the two tissues are similar in terms of their subunit and native molecular weights and kinetics with respect to both malate and NADP+. Antibodies to the two enzymes cross react with each enzyme. A single radial immunodiffusion assay showed a linear relationship between the activity and amount of enzyme protein in both mammary or liver preparations suggesting an immunochemical identity for the two enzymes. This assay was used to show that the greater than 5-fold change in activity which occurs during the lactation cycle is a consequence of differences in the amount of enzyme protein rather than any activation of a preformed enzyme.