Little information is available about porin molecules in anaerobes. Porins from Bacteroides fragilis and Porphyromonas, Fusobacterium, and Campylobacter species have been described. A pore-forming outer membrane (OM) porin protein was isolated from B. fragilis (Omp-200); it is exposed at the cell surface and dissociated by boiling and application of reducing agents. Fusobacterium nucleatum FomA, an OM porin protein of 40 kD, had a deduced topology of FomA similar to that of established porins, despite the lack of sequence similarity. An OM preparation from Porphyromonas endodontalis (including a major protein with an apparent molecular mass of 31 kD and other proteins of 40.3-71.6 kD) formed pores in a liposome assay. A major outer membrane protein (MOMP) from Campylobacter jejuni (a microaerophile) is related to the family of trimeric bacterial porins, although little homology was seen with other porins. The development of antimicrobial resistance related to decreased permeability underlines the importance of identifying and characterizing the pore-forming molecules of anaerobes.