Studies of the mechanism of experimental nephrotoxic nephritis (1-8) have been concerned largely with: (a) the specificity of the nephrotoxins, (b) their sites of localization, especially in the kidney, and (c) the presence and the role of immune responses in the nephritic animal. In recent years a considerable body of direct evidence relating to these problems has been furnished by the use of labelled antibody techniques, first applied to this field in the form of radioiodinated nephrotoxins by Pressman et al. (4,5,9).Among the results achieved by this group was the demonstration that nephrotoxic T-globulins are preferentially absorbed by the kidney, where they localize, at least in greater part, in the glomeruli, and that the radioactive tracer and, presumably, the intact antibody persist for up to 2 months at the sites of tissue localization. Recently, the potentialities of tracer methods have been more fully realized with the introduction by MeUors et al. (10) of fluorescein-coupled antibodies as histochemical reagents for the detection of tissue-localizing antibodies in viva. This new application of the technique developed by Coons et al. (11,12) not only makes it possible to define the sites of localization of nephrotoxic antibodies with a hitherto unattainable precision, but for the first time it permits the demonstration of deposits of autogenous antibodies as well. A highly advantageous characteristic of the technique is that it detects antigenic and, therefore, relatively intact antibodies only.In a series of papers on experimental and human nephritis studied by this method, Mellors et al. (10,13,14) have unequivocally confirmed the selective glomerular localization of nephrotoxins in the rat kidney, indicating also that it is primarily membranous, and they have demonstrated the presence of glomerular localizing autogenous antibodies in a foreign protein nephritis of rabbits and in several types of human nephritis. The present investigation is an extension of the work dealing with nephrotoxic nephritis in the rat.