Antibody activity for various antigens has been found chiefly in the yG (7s) and the yM (19s) fractions of globulin. The molecular type of antibody which predominates appears to depend to some extent on the chemical nature of the antigen involved. Protein antigens such as serum albumin and bacterial flagella induce the early formation of yM antibody followed by yG which soon comprises the major portion of the antibody( 1 ) . Other antibodies, notably those produced in response to the somatic antigens of Gramnegative bacilli (1,2,3) and the infectious mononucleosis antibody that reacts with the beef thermostable antigen (4), are predominantly or exclusively in the yM globulin.Since only limited information is available regarding the antigenic composition of leptospirae and the relation of antigenic components to various serologic reactions involving these organisms ( 5 ) , it seemed of interest to determine the types of globulin associated with the agglutinins in antileptospira sera.Materials and methods. Rabbits were immunized by 4 intravenous injections, each consisting of 1 ml of live culture of leptospirae, given during a period of 6 days. Three of the sera from patients with leptospirosis had been preserved in the frozen state for 6 to 7 years without apparent reduction in agglutinin titer. Serum from one patient, 812, was examined after 6 weeks refrigeration without being frozen. Antibodies were determined by the microscopic agglutination test employing live cultures in Korthof's medium( 6). Chromatographic separation of fractions from 5 ml samples of serum was accomplished as previously described (3,4) or, when the amount of serum was limited, 1 ml of serum was placed on a column containing 1 g of DEAE cellulose and from 8 to 20 4-ml fractions were eluted with each *This work was supported by a grant from National Science Foundation.of the 4 buffers. Individual fractions were screened for agglutinins and the positive fractions eluted witheach buffer were pooled and concentrated with the aid of carbowax(3). Sensitivity of antibody to disulfide bond reduction was determined by mixing equal volumes of diluted serum and 0.2 M 2-mercaptoethanol (ME), allowing the mixture to stand at room temperature for 16 to 18 hours, and testing for agglutinins without further treatment. A control consisting of equal parts of serum dilution and saline was included in each instance.Results. Human sera. The data obtained on analysis of serum specimens from 4 patients with leptospirosis are shown in Table I. Each serum reacted with at least 4 serotypes. If one assumes that the type showing the highest agglutinin titer was the type responsible for the infection, it appears that patients 328 and 812 were Leptospira canicola infections, that patient 336 was infected with L . pomona, and that L. grippotyphosa was the infecting type in patient 406. Although the differences between titers for L. sejroe and the presumed infecting type was only one tube, it is unlikely that all 4 patients had multiple infections, especially since neithe...