Studies are in progress on cytochromes c3 from the sulfate-reducing bacteria, a unique four-electron acceptor of negative redox potential. EPR determinations of mid-point potentials and n-values have been made for cytochrome c3 from Desulfovibrio vulgaris and Desulfovibrio gigas. The four hemes of these cytochromes were found to be n equals 1 systems with mid-point potentials varying between -235 mV through -324 mV, considerably more negative values than previously reported. A mutant from Azotobacter vinelandii has been obtained by mutagenic treatment (AV-11). This mutant is oxidase-negative but contains almost as much cytochrome d as the wild-type organism. Cytochrome d has altered properties as does cytochromes c4 plus c5. Site III in this organism is defective; this y system will be used as a model system for energy conservation and the mechanism thereof. ATPase has been solubilized and partially purified from A. vinelandii to a specific activity of nearly 40 micromoles phosphate per min per mg protein. After purification to homogeneity by the three step chromatographic procedure now utilized, the interaction of trypsin with the control inhibitor subunit will be examined.