Malic enzyme from leaves of the Crassulacean species Bryophyllum tubiflorum was purified more than 1200-fold by a rather simple procedure involving repeated fractionation with ammonium sulphate and gel-filtration. A specific activity (international standards) of 68 was obtained. The molecular weight of the enzyme, measured by the gel-filtration method, was 237000.Thiol activation is required for catalytic activity. The activity is completely dependent on divalent cations; greatest activity was obtained with Mn2+, an equimolar amount of Mg2+ was 86O/, as effective. Maximum activity was obtained at about pH 7.2. At 30 "C and pH 7.2 the K , for Mn2+ was 0.46 mM, that for NADP was 14 pM, and that for malate amounted to 0.37 mM. The K , for malate was temperature-dependent : it remained constant in the temperature range from 17 tco 39 "C, but below 17 "C and beyond 39 "C the affinity for malate decreased and a Kmvalue of 0.94 mM was recorded. The enzyme was completely NADP-specific and was stable at temperatures up to 55 "C; maximum activity was recorded at 50 "C.No effects of monovalent cations such as Naf, K+, Li+, or NH,+ could be recorded. The enzyme was inhibited by coenzyme A and thiamine pyrophosphate, but not by acetyl-coenzyme A. The effects of temperature as well as the inhibition by coenzyme A and thiamine pyrophosphate fit the physiology of Crassulacean acid-metabolism.Crassulacean leaves show a diurnal fluctuation in acid content, i.e. the amount of (predominantly) malic acid rises at night and decreases in daytime. This phenomenon has been called Crassulacean acidmetabolism [ 11. To phosphoenolpyruvate carboxylase a role was attributed in the acidification reactions, while malic enzyme was active in the deacidification reactions [ 2 ] . This kind of malate metabo h m in Crassulaceae may be a variant of the C,-dicarboxylic-acid pathway of GO,-fixation in (efficient) photosynthesis [3].