Cytochrome b 2 from baker's yeast was found to contain, for each 230,000 g. protein, one FMN residue, one heme group and eight iron atoms not bound to heme. These iron atoms were not detached by dialysis against o-phenanthroline, 8-hydroxyquinoline and N,N′-dihydroxyethylglycine, both in the presence and in the absence of lactate. The enzyme gives in the presence of lactate and o-phenanthroline an active red compound. Different experiments demonstrated the absence of a DPN dependence of cytochrome b 2. The lactic dehydrogenase activity of cytochrome b 2 is reversible, FMNH 2 and pyruvic acid acting as substrates under anaerobic conditions. The maximal rate of reduction of ferricytochrome c and ferricyanide in the presence of cytochrome b 2 and excess lactate was 14,000 moles/mole enzyme/min. The analogous figure for methylene blue was 2200. Quinones are also reduced at a very high rate (14,000 moles/mole/ min.). This quinone reductase activity is higher than that known for the DPNH-dependent quinone reductase of pea seeds. Cytochrome b 2 has a group dissociating with p K = 5.65 and n = 1 in either the reduced or the oxidized form (probably in the former). Salts inhibit the activity of cytochrome b 2 when methylene blue or ferricytochrome c are used as the oxidants. Quinones prevent this effect.