Low-temperature e.p.r. (electron-paramagnetic-resonance) spectroscopy was used to detect electron-transport components in Chromatium chromatophores with e.p.r. signals in the g=2.00 region. High-potential iron protein (Em8.0=+325mV, where Em8.0 is the midpoint potential at pH8) and a second component (g=1.90, Em8.0=+285mV) are oxidized in illuminated chromatophores. Two iron–sulphur proteins (g=1.94) with Em8.0=−290mV and Em8.0=−50mV are present. One (Em8.0=−50mV) is reduced on illumination. A component (g=1.82) with Em8.0=−135mV is photoreduced at 10°K. The midpoint potential of this component is altered by o-phenanthroline and pH. The properties of this component suggest that it is the primary electron acceptor of a photochemical system. Another component (g=1.98) also has some of the properties of a primary electron acceptor, but its function cannot be completely defined. These results show that iron–sulphur proteins are present in the electron-transport system of Chromatium and indicate their role in electron transport.