We have characterised a protein from Bordetella pertussis, the whooping cough agent, and found it to be almost identical with the heat shock protein from E.coli, GroEL. These molecules, also termed chaperonins, are oligomeric proteins of approximately MW800.000 comprised of 14 subunits arranged in two rings of 7 subunits each, with a diameter of 15nm. When negatively stained, the GroEL-like molecules, examined in the electron microscope, are seen mainly in the top view clearly displaying the sevenfold symmetry, or as the less common side view with three characteristic electron dense striations separating four stain excluding regions (Fig. 1).