Abstract Reduced lipoamide dehydrogenase reacts with cadmium ion to form a moderately stable compound characterized by 1 g atom of cadmium bound per mole of flavin adenine dinucleotide, a modified spectrum in the visible region, and altered reactivity with electron acceptor substrates. The 450-mµ band is shifted to the blue and the 370-mµ shoulder is lost. On storage, the spectrum of the cadmium-lipoyl dehydrogenase complex tends to return to that of the unmodified enzyme. Alkylated lipoyl dehydrogenase, inactive in the DPNH-lipoamide dehydrogenase reaction, forms the corresponding cadmium compound. The latter appears more stable than the cadmium compound of the unalkylated enzyme. The activity profile of the cadmium compound is characterized by increased rates of 2,6-dichloroindophenol and quinone reductase, little change in ferricyanide reductase, decreased pyridine nucleotide transhydrogenase, and highly inhibited DPNH-lipoamide dehydrogenase activity.