Morphofunctional and immunofluorescent studies were done in guinea pigs experimentally infected with Leptospira interrogans serovar icterohaemorrhagiae to determine the role of leptospiral antigen, gamma-globulin, and complement C3 deposits in the pathogenesis of the renal lesions. A sharp increase in leptospiral antigen deposits was observed in the late phase of the experimental infection. Immunoglobulin and complement C3 were small compared with the heavy leptospiral antigen deposits. Moreover, leptospiral antigen was observed mainly in the interstitium whereas immunoglobulin and complement C3 deposits were seen in glomeruli and small blood vessel walls. In our experimental model bacterial migration and local liberation of factors causing virulence seems more likely to determine the renal damage.