Detection of leptospiral antigen using an immunoperoxidase staining (IP) procedure was carried out on fifteen samples of human liver, (nine from autopsies and six from biopsies) and nine samples of human kidneys (eight autopsies and one biopsy). The IP staining procedure to detect leptospiral antigen (L. interrogans serovar icterohaemorrhagiae) in human liver and kidney proved to be a reproducible method useful on paraffin embedded tissues after formalin, Bouin's or Helly's fluid fixation. Furthermore, the IP procedure on paraffin-embedded tissue appears to have potential as an aid to the diagnosis. IP stained leptospiral antigen was detected in portal spaces of the liver, engulfed by cells of the mononuclear phagocyte system, in the interstitium of the kidney, and lining vessel walls of both liver and kidney. The results suggest that in acute human leptospirosis (L. interrogans serovar icterohaemorrhagiae) the main factors in the pathogenesis of the lesions are related to the presence of organisms and/or their virulence, including products released by lysis.