Quantitative crossed immunoelectrophoresis was used to evaluate the antigenic similarity of Pseudomonas aeruginosa and Pseudomonas cepacia GroEL proteins. We found that the two proteins showed 75% identity. By using a panel of monoclonal antibodies against the P. aeruginosa GroEL protein, we identified 10 monoclonal antibodies which cross-reacted with the P. cepacia GroEL protein and 21 monoclonal antibodies which recognized type-specific epitopes on the P. aeruginosa GroEL protein. In crossed immunoelectrophoresis two different fractions of GroEL reactive material could be resolved. These fractions showed a reaction of partial identity. Examination of the two immunoprecipitates by Western blotting, showed that both fractions consisted of anti-60 kDa GroEL reactive protein. One fraction, in addition, contained LPS with a characteristic 'ladder' reaction in modified Western blotting. We therefore conclude that this fraction represents a complex between LPS and GroEL.