Injection of rabbit anti-rat thymocyte serum (ATS) i.v into rats induces a transient glomerulopathy characterized by immune aggregates localized in the mesangium and along the glomerular capillary wall, as detected by immunofluorescence (IF) techniques. Neither light microscopical alterations in the kidney, nor proteinuria could be detected in these animals and no autologous IgG could be observed in the glomeruli during the observation period (45 days). Results from ex vivo perfusion studies showed identical localization of immune aggregates—which, in electron microscopy, appear to be localized subepithelially. The ATS used, was monospecific in that no other specificities could be detected after thorough absorption with rat tissue extracts including tubular brushborder antigens, so it is concluded from these data that ATS is able to participate in the formation of immune complexes in situ by recognizing epitopes both in the mesangium and at the epithelial side of the glomerular basement membrane.