Lipase and antilipase were studied in the following genera: Serratia, Aeromonas and Vibrio. Numerous comparative investigations showed that the Jipase of Serratia marcescens is not of a uniform antigenic structure as not all antilipase-sera of Serratia marcescens have the ability to neutralize the lipase of all studied strains of this species. Cross-reactions were seen between the genera Serratia and Aeromonas. A special reaction of the antilipase sera of Aeromonas hydrophila and Aeromonas punctata could be demonstrated on the lipase of one strain of Serratia marcescens. Those Aeromonas sera could neutralize the lipase of one strain of Serratia marcescens.