Studies have been conducted with the arginase (l-arginine amidinohydrolase, EC 188.8.131.52) of two legumes: jack bean, Canavalia ensiformis (L.) DC., a l-canavanine-containing plant and soybean, Glycine max, a canavanine-free species. Analyses of the arginase obtained from gradient-purified mitochondria of these legumes revealed that the arginine-dependent (ADA) and canavanine-dependent activities (CDA) were localized within this organelle.Kinetic analyses of affinity-purified mitochondrial arginase revealed an apparent K(m) of 7 to 8 millimolar for arginine with both the jack bean and soybean arginases. Comparable determinations with canavanine revealed an apparent K(m) of 38 millimolar with the jack bean enzyme; the affinity for this arginine analog with the soybean enzyme is so poor that product formation remained linear even with a canavanine concentration of 890 millimolar.A single macromolecule appears to be responsible for both the ADA and CDA of jack bean arginase. Ion-exchange chromatography of mitochondrial arginase revealed that the ADA and CDA eluted as a single, discrete peak from DEAE-cellulose. Analyses with arginine- and canavanine-linked Sepharose failed to reveal more than one enzyme. Both the ADA and CDA increased by nearly identical amounts following elution from arginine- and canavanine-linked cyanogen bromide-activated sepharose. Neither ADA nor CDA increased preferentially over the other.