Intravenous administration of rabbit anti-rat thymocyte serum (ATS) reactive with Thy-1-like antigens present on rat mesangial cells induces almost immediate (1-hr) mesangial cell injury in rats followed by sequential mesangiolytic and mesangial-proliferative/infiltrative lesions. To determine the role of complement in these ATS-induced glomerular lesions, ATS was given to Lewis rats that had been depleted of C3 by cobra venom factor (CVF). CVF treatment prevented the degenerative changes in mesangial cells and accumulation of even the few polymorphonuclear leukocytes (PMN) seen in the glomeruli (2.67 PMN/glomerulus) 1 hr after ATS-treatment in rats not given CVF. In addition, CVF prevented the mesangiolysis and mesangial hypercellularity seen at day 4. Rat C3 and late complement components identified in the mesangial of ATS-treated rats in close association with the deposition of rabbit immunoglobulin G was also absent as a result of CVF treatment. CVF treatment did not affect binding of ATS to glomeruli as studied by immunofluorescence or paired label radioisotope techniques. The depletion of leukocytes and/or PMN by irradiation or treatment with anti-I-MN serum had no effect on the induction of the acute mesangial cell damage or the mesangiolytic lesion. Irradiation did diminish the 4-day proliferative/infiltrative lesion. Complement depletion normalized the ATS-induced increase in mesangial uptake of heat-aggregated human gamma-globulin (655.0 +/- 35.2 micrograms in ATS-treated vs 20.3 +/- 2.9 micrograms/5 X 10(4) glomeruli in ATS plus CVF-treated rats; mean +/- SEM). Small immune deposits present in the mesangial areas of kidneys 4 to 5 days after CVF treatment represented CVF-anti-CVF antibody-C3 complexes. The model of mesangial cell damage induced by ATS in the rat is complement-dependent and may relate, at least in part, to complement-mediated mesangial cell lysis.