The major outer membrane protein (MOMP) of Chlamydia shares several biochemical properties with classical porin proteins. To directly test the "porin channel" hypothesis at the molecular level, MOMP was reconstituted into planar lipid bilayers where it gave rise to "triple-barrelled" channels which were modified by an anti-MOMP neutralising monoclonal antibody. These observations are consistent with the well characterised homo-oligomeric nature of MOMP previously revealed by biochemical analysis, and the "triple-barrelled" behaviour of other porins. MOMP channels were weakly anion selective (P_Cl/P_K ~ 2) and permeable to ATP. They may therefore be a route by which Chlamydia can take advantage of host nucleoside triphosphates, and explain why some anti-MOMP antibodies neutralise infection. In order to undertake more detailed studies of the MOMP structure/function relationship, recombinant MOMP from both C. psittaci and C. pneumoniae have been cloned and expressed. The recombinant proteins were functionally reconstituted in planar lipid and analysed at the single channel level. Both form porin-like ion channels that are functionally similar to the native protein. The C. psittaci recombinant porin was modified by the same anti-MOMP neutralising monoclonal antibody that effected the native protein. In contrast to the native protein, both recombinant C. psittaci (P_Cl/_K ~ 0.38) and C. pneumonia (P_Cl/P_K ~ 0.49) proteins were marginally cation selective. This is the first time native function has been demonstrated for recombinant chlamydial MOMP and will have an important impact on the future development of subunit vaccines.