Organized particulate structures (rods) identified in purified preparations of exotoxin A from culture supernatants of Pseudomonas aeruginosa PA103 were found to be immunochemically cross-reactive with exotoxin A. The rods were visualized by electron microscopy after negative staining as hollow tubes or sheaths (45 by 15 nm). Purified rods were not toxic and not enzymatically active in the ADP-ribosylation assay. Antigenic cross-reactivity between exotoxin A and rods was demonstrated by using monoclonal antibodies directed against either rods or a toxoid of exotoxin A. Hybridoma clones derived from mice immunized with rods or toxoid reacted with both antigens in the enzyme-linked immunosorbent assay. Rods could be dissociated by boiling and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into three subunit polypeptides with molecular weights of 70,000, 45,000, and 27,000. Two of the three subunit polypeptides reacted both with antirod and antitoxin monoclonal antibodies after electrophoretic transfer of sodium dodecyl sulfate-polyacrylamide gel electrophoresis-separated proteins to nitrocellulose filters. The results indicate that rods and exotoxin A share common antigenic determinants.