The ultraviolet visible, and near infrared spectrum of a two-iron protein from Desulphovibrio gigas, a new type of non-haem iron protein lacking labile sulphide, is compared with that of D. gigas rubredoxin. The charge transfer band maxima of rubredoxin at 495 and 565 nm are less separated in the new protein implying a higher symmetry of the two iron centres. The existence of a spin-spin interaction between the two iron centres in the new protein is suggested by the magnetic susceptibility measurements of the oxidized and reduced states of both proteins, which gives a smaller value per iron centre for the new protein. The oxidized form of the two iron-protein has a complex EPR spectrum with signals at g values of 8.97, 7.72, 5.73, 4.94, and 1.84. An EPR titration gives a value of --35 +/- 15 mV for the two signals at g values of 7.72 and 5.73. Rubredoxin has the characteristic spectrum of rubredoxins with two signals at g values of 9.4 and 4.27.