Exposure of Anabaena 7120 cells to membrane perturbant such as EDTA or Tris (pH 8.0, 37.degree. C, for 5-10 min) resulted in the release of outer membrane lipopolysaccharide and proteins from cells. After Tris treatment, the sensitivity of cells to crystal violet and detergents such as SDS and Triton X-100 increased and whole-cell alkaline phosphatase activity enhanced obviously, suggesting that the structure of outer membrane was modified and its permeability increased. At the same time. Tris was found to reduce nitrogen fixation activity of cells considerably in air, but not in anaerobic condition. Reconstitution of Tris-treated cells with released material might recover nitrogen-fixing activity of cells clearly, indicating that the structure of outer membrane is closely related to the protection of nitrogen fixation from oxygen. Although EDTA-treated cells released more lipopolysacharide and proteins than those of Tris-treated cells, the permeability of outer membrane and nitrogen-fixing activity were not influenced significantly. SDS-gel electrophoresis showed that Tris-treated cells released 3-4 specific polypeptides which were not present in the released material from EDTA-treated or water-treated cells. These experiments suggest that membrane perturbants-induced loss of outer membrane function is mediated through the modification of specific position in outer membrane.