The outer-membrane protein (OMP) profiles of four isolates of Haemophilus paragallinarum (0083, 0222, Modesto, and HP31) were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. OMPs were isolated by sonic disruption followed by differential centrifugation and selective solubilization in Triton X-100. Although the isolates had similar profiles overall, two distinct OMP profile types, based on the variable molecular weight of a protein termed OMP C (39,000 or 38,000), were found. In addition, OMP C was found to be a heat-modifiable protein--being either absent or present in only minor amounts if the preparations were not heated at 100 C. Major and minor OMPs, some common to all four isolates, were recognized in immunoblots by an immune serum to isolate HP31.