MWapp 116,000 and 29,000 proteins (p116 and p29), major outer membrane proteins of Hyphomonas jannaschiana reproductive cells, were extracted from cell envelopes by dialysis against EDTA, 2 M urea or distilled water. These proteins were precipitated by divalent cations and resolubilized by EDTA-Na, reflecting alternate monomer, multimer states. From two-dimensional gel electrophoresis it was determined that p116 and p29 had a pl of 4.5. Both were glycoproteins. Results suggest that p116 and p29 are surface layer (S-layer) proteins, with p116 a tetramer of the p29. The S-layer could protect the adherent H. jannaschiana reproductive cell from exoenzyme activity, antibiotics and other bacteriocidal molecules produced in the bacterial films formed on many marine surfaces.