Two proteins with apparent molecular weights of 39,000 and 36,000 (M(r) 39,000 and M(r) 36,000, respectively) were isolated from the outer membrane of Proteus mirabilis 19. M(r) 36,000 was shown to be free of detectable amounts of the M(r) 39,000 protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and free of lipopolysaccharide according to gas chromatographic analyses of 3-hydroxymyristic acid content. The M(r) 39,000 protein contained no detectable amount of lipopolysaccharide and only a trace of M(r) 36,000. Both isolated proteins gave strong reactions in antisera produced to purified P. mirabilis 19 cell walls (outer membrane proteins in the native state). This suggested that the proteins isolated by our methods essentially retained their native configuration upon resolubilization. Antisera produced in rabbits to the isolated proteins showed strongest reactions with the homologous antigen, but some cross-reactions with the heterologous protein and with P. mirabilis 19 lipopolysaccharide were observed. These cross-reactions could be attributed to specific responses to traces of the heterologous (contaminant) proteins present in the purified proteins used as immunizing antigens. The M(r) 39,000 and M(r) 36,000 proteins have no major antigenic determinants in common. Reactions with P. mirabilis 19 lipopolysaccharide in antisera to the outer membrane proteins could be completely removed by absorption of the antisera with the M(r) 36,000 protein.