To analyze the characteristics of Leptospira strains HY-1, HY-2, and HY-10, which were isolated from patients with leptospirosis in Korea in 1985, 12 monoclonal antibodies (MAbs) against strain HY-1 and six MAbs against serovar lai strain 017 were produced, and their properties were determined by the microscopic agglutination test. Genetic relationships among the leptospires were determined by restriction endonuclease DNA analysis. Three MAbs reacted with all strains of serogroup Icterohaemorrhagiae, but did not react with any strains of the other 10 serogroups. All MAbs reacted with strains 017, HY-1, HY-2, and HY-10 at nearly identical titers. Two MAbs reacted only with these four strains. These four strains also had the same restriction endonuclease cleavage patterns. Based on these results, strains HY-1, HY-2, and HY-10 were identified as serovar lai, which is one of the common serovars in China. It is suggested that serovar lai is one of the prevalent serovars in Korea, and that the Mabs produced in this study are useful for the accurate and rapid identification of this serovar.