ELISA was used to detect specific IgM and IgG in sera from humans with current or past leptospirosis. A serological pattern of a high IgM titer (.gtoreq. 1280), or moderately increased IgM (160-640) in conjunction with a low IgG titer (.ltoreq. 20), with serovar copenhageni antigen was characteristic for .apprx. 2/3 of the sera from serovar icterohamemorrhagiae patients obtained in the first 2 mo. of the disease. The antigen was the supernatant of a heated and centrifuged culture of leptospires. Antigens were prepared from serovars copenhageni, grippotyphosa, hardjo and patoc. Sera from patients with icterohaemorrhagiae, grippotyphosa and hardjo infections showed cross-reactivity when different antigens were used. In past infections the IgG titers were clearly higher with the homologous antigen. ELISA for IgM and IgG allows the rapid diagnosis of acute leptospirosis.