Photon correlation spectroscopy demonstrated for the first time that co-purified meningococcal TbpA+B form a complex in solution. This structure bound hTf and the resultant species under,vent partial dissociation after exposure to additional hTf or following prolonged incubation. Purified TbpA and TbpB had similar apparent sizes but showed distinctive size profiles suggesting that TbpA forms a largely homogeneous population while TbpB may produce more variable particle sizes under these conditions. (C) 1997 Federation of European Biochemical Societies.