Agglutination with serotype patoc (Patoc 1) was found in 72 of 125 cases of leptospirosis. Patoc-agglutination occured frequently in leptospirosis icterohaemorrhagiae, less frequently in leptospirosis grippotyphosa and rarely in leptospirosis tarassovi. 53 cases of leptospirosis were not diagnosed when using Patoc 1 as single antigen. So we cannot recommend to use serotype patoc as the sole antigen for microscopic agglutination test. We observed 1 case in which the reaction with Patoc 1 happened earlier than the reaction with the pathogen serotypes. For this reason the battery of antigens for carrying out microscopic agglutination test should include serotype patoc, too.