Abstract Our prior investigation of Chlorella malate dehydrogenase (MDH) revealed supernatant and particulate isoenzymes which were immunologically, chromatographically, and electro-phoretically distinct. By means of ammonium sulfate fractionation and column chromatography a crystalline preparation of the particulate isoenzyme, specific activity of approximately 2000 has been obtained. It appears homogeneous in the analytical ultracentrifuge, by column chromatography and by electrophoresis. This preparation migrates as a single band of activity when subjected to starch gel electrophoresis and its mobility and activity (on the gel) are unaffected by prior incubation with citric acid, pH 2.0, EDTA, β-mercapto-ethanol, substrates or products; however, new bands appear when preincubated with exogenous proteins (RNase, Ovalbumin, Aldolase).