Rabbit antisera were prepared against living cells of six different ammonia oxidizing nitrifying bacteria. They were examined as to cross-reactivity in the agglutination test (Microtiter-system) with 24 nitrifier strains, including members of all known genera. Usually distinct cross-reactions were obtained only within the genera, but some exceptions were noticed. There was stated a clear cross-reaction between the two anti-Nitrosospira-antisera and the four tested Nitrosolobus strains. In some cases cross-reactions between cells of the Nitrosovibrio strains and the anti-Nitrosospira- as well as the anti-Nitrosococcus-antisera could be observed. The interpretation of the results obtained with the Nitrosomonas group was complicated by the fact that all strains showed positive zero titers with the control sera. In seven cases lipopolysaccharides were isolated and tested in the passive hemagglutination test to their cross-reactivity with the above mentioned antisera. Hemagglutination could only be observed in the homologous system, cross-reactivity was never expressed.