In an attempt to define the pathogenetic mechanisms involved in acute interstitial nephritis due to Leptospira canicola the kidneys of 14 dogs were subjected to histological immunofluorescence and elution studies. Leptospiral antigen was found in three sites within the kidney; as discrete organisms in the proximal tubules, as granular deposits in the cytoplasm of macrophages and in large extracellular clumps in the interstitium. Deposits of IgG were also detected in the latter site and in plasma cells found in the surrounding cellular infiltrate. Anti-leptospiral antibody, mainly of the IgG class, was detected in renal eluates using both agglutination-lysis and indirect immunofluorescence. From these findings it was considered that the cellular infiltrates found in the kidneys of dogs suffering from acute leptospiral nephritis have two main functions; firstly, the local production of anti-leptospiral antibody and, secondly, phagocytosis.