1. The molybdenum-iron (Mo-Fe) protein, iron (Fe) protein and the activating factor of nitrogenase from Rhodospirillum rubrum were purified. 2. The Mo-Fe protein has properties similar to those of the Mo-Fe proteins of other nitrogen-fixing organisms. 3. The Fe protein is similar to other Fe proteins with respect to its molecular weight, metal composition and e.p.r. signal. 4. The Fe protein is different from other Fe proteins in that it apparently has two types of subunits rather than one, its u.v. spectrum has an extra peak, and phosphate, ribose and an adenine-like unit are covalently bound to the protein. The presence of these non-protein groups on the protein may explain the requirement for activation of R. rubrum Fe protein.