The polypeptide compositions and antigenic components of Rickettsia tsutsugamushi were analyzed by modifying the solubilization conditions prior to polyacrylamide gel electrophoresis and by using monoclonal antibodies in immunoblotting experiments. Several polypeptides were converted to larger or smaller molecules by using various conditions for rickettsial sample preparation. Solubilization of a sample in 2-mercaptoethanol-containing buffer resulted in conversion of high-molecular-weight polypeptides to smaller polypeptides and conversion of some of the 43-kilodalton (43K) polypeptide to a 46K polypeptide. The heat modifiability of selected polypeptides was shown by heating samples at 100 degrees C. A major polypeptide on the rickettsial surface which showed strain-specific antigenicity appeared at the 43K position in samples solubilized at 37 degrees C but moved to the 56K position after samples were heated at 100 degrees C. Immunoblotting with an anti-56K polypeptide monoclonal antibody demonstrated that the reactive antigens existed predominantly as the higher-molecular-weight polypeptides. These polypeptides were converted to 43K polypeptides at 37 degrees C or the 56K polypeptides at 100 degrees C by cleavage of disulfide linkages with 2-mercaptoethanol treatment.