Using the model of renal disease induced in guinea pigs by immunization with bovine TBM preparations in adjuvant, the following observations were made. Animals with activity induced disease show bright staining for IgG along the TBM and only faint, inconstant staining along the GBM. Following transfer of serum animals with anti-TBM disease to normal recipients, accumulation of IgG was found predominantly in glomeruli at 4 hours, but at Days 3 and 5, IgG was seen predominantly along the TBM. There was no appreciable accumulation of neutrophils in the kidneys of recipients of anti-TBM serum, even at early intervals (4 and 24 hours) after transfer. However, within 2 days, small numbers of mononuclear cells were found. By Day 3, mononuclear cells were numerous, and multinucleate giant cells and tubular cell damage were present. After that, the lesions increased in severity and by 10 days were indistinguishable from those found in actively immunized animals at 14 to 21 days. Study of frozen section of kidneys obtained from animals with active disease at 14 days, employing sheep cells coated with rabbit antibody (IgG EA) revealed rosettes around many of the mononuclear cells in the infiltrate, indicating that they are mononuclear phagocytes (monocytes or macrophages). IgM complexed with sheep cells and complement (EAC) did not react and thus failed to provide evidence for the presence of B lymphocytes. Transfer of 7 X 10(8) lymph node cells from the TBM-immunized Strain 13 donors to normal Strain 13 recipients failed to result in renal lesions. The findings are interpreted as indicating that anti-TBM antibodies mediate the renal disease without the participation of cell-mediated immunity and further that these antibodies bring about an influx of ciculating mononuclear cells, predominantly monocytes, without attracting appreciable numbers of neutrophils.