SUMMARYSerum samples from patients infected by organisms of Leptospira interrogans serogroup Australis were tested by Western blot to determine the nature of major antigens that are involved in the immune response. Although there was some patient-to-patient variability, immunodominant genus-specific antigens were found to be proteins of apparent molecular ratio 68, 46 and 35-kDa, and lipopolysaccharide (LPS) sub-units in the 35-14-kDa region. Serogroup epitopes specific for Australis were exclusively saccharides of about 32 and 24 kDa: a serovar-specific antigen for serovar lora was of 38-40 kDa and behaved like a protein. Antibodies to the LPS serogroup-specific antigens and to the 38-40 kDa protein were long-lasting and consequently suggest that these Correspondence to: