H-specific agglutinating sera to Pseudomonas aeruginosa were prepared by immunisation with partially purified flagella. The results of agglutination and immobilisation tests with rabbit sera prepared against the flagella of six H-type strains showed that the sera had high titres and were H specific. Cross-absorption tests revealed that one strain (H-3) possessed a distinct antigen not present in any of the others. Two groups of strains (H-1, H-2 and H-5; H-4 and H-6) each possessed a distinct major antigen. Members of these two groups could be distinguished by their minor antigens.