106 selected kidneys removed at autopsy were studied by direct immunofluorescence using polyvalent antisera against human immunoglobulins, light chains, complement fractions and fibrinogen. The immunofluorescence was a suitable method to solve differential diagnostic problems that arose at autopsy. The diagnostic value was the most obvious in cases of immunologically mediated renal diseases and in immunologically mediated systemic diseases involving the kidneys. Negative immunofluorescence findings were also useful to determine the pathogenesis of renal lesions, especially in vasculopathies. The immunofluorescence of postmortem material showed similar disturbances to that obtained with biopsy material. At various sites, especially in the tubulo-interstitium, additional electron microscopical study was sometimes needed to localise the immune deposits exactly. The fluorescent microscopical examination of frozen sections of kidney taken at necropsy turned out to be more adequate than the immunoperoxidase examination of formalin-fixed, paraffin-embedded sections.