In order to analyze the serological characteristics of Leptospira interrogans serogroup Icterohaemorrhagiae, monoclonal antibodeies were produced against the serovar smithi Smith strain and the serovar naam Naam strain by cell fusion technology and their properties were examined.Initially, 18 anti-Smith monoclonal antibodies (SMIMA) and 4 anti-Naam monoclonal antibodies (NAAMA) were made, and their serological properties examined by microagglutination test (MAT). Consequently, SMIMA-1 and NAAMA-1, reacting only with homologous strains, were made. Some antibodies recognized common antigen (s) widely distributed among the serogroup Icterohaemorrhagiae except for serovars weaveri, sarmin and tonkini; other antibodies showed different reaction patterns. Next using these monoclonal antibodies, the serological relationships existing among the leptospira serovars of the serogroup Icterohaemorrhagiae were examined. These were divided into three subgroups as follows (1) Serovars monymusk, birkini, mankarso, icterohaemorrhagiae, copenhageni, ndambari, ndahambukuje and budapest, which showed cross-reactivities with the serovar smithi, immunogen leptospira. (2) Serovars weaveri, sarmin and tonkini, which showed no crossreactivities with the serovar smithi. (3) Serovars gem, bogvere, mwogolo, naam and dakota, which showed intermediate characteristics. Also, in subgroup 1, serovars smithi, monymusk, birkini, mankarso, icterohaemorrhagiae and the copenhageni Shiromizu strain showed similar reaction patterns. Serovars copenhageni M20 and Shibaura strains, ndambari, ndahambukuje and budapest showed similar reaction patterns. Serovars monymusk and birkini showed strong cross-reactivities with serovar naam, while serovar icterohaemorrhagiae showed weak cross-reactivity. The antigenic determinants of leptospires were analyzed by the immunoblotting technique using monoclonal antibodies, and were found to react with 18 monoclonal antibodies within a range of molecular mass of approximately 23 kD irrespective of their serological characteristics. The 23-kD molecular mass band was not stained with Coomassie blue but did show a reaction upon silver staining. The band was heat-and proteinase K-resistant, but it was degraded upon treatment with sodium metaperiodate. Accordingly, the band seemed to be a lipopolysaccharide (LPS).