Serogrouping of P. aeruginosa on the basis of heat-stable somatic (O)-agglutinogens is well established, but H-typing on the basis of heat-labile agglutinogens is not accepted generally because of uncertain flagella specificity. In contrast to the agglutination reaction, the indirect fluorescent antibody technique allows the differentiation of soma and flagellum of P. aeruginosa cells at the morphological and serological level simultaneously, and thereby the specific analysis of flagellar (H)-antigens, even with OH-antisera. Using this technique the complex flagella-antigen a with the partial antigens a0, a1, a2, a3, a4 and the uniform flagella-antigen b are differentiated by cross absorption experiments. As the partial factors a1-a4 are independent determinants, a flagellar antigenic schema containing 17 H-types is devised. Type strains, immunization and absorption procedures for preparing diagnostic antisera are described. O-group and H-type show free combination. Completing the extended O antigenic schema of Habs (slide agglutination) with the flagella specific H antigenic schema (indirect fluorescent antibody technique), a detailed OH typing system of P. aeruginosa is obtained, by which clinical isolates are subdivided into 99 serovars.