An immunofluorescent study of renal biopsies from patients with toxemia of pregnancy has been performed. It was found that the glomeruli consistently showed bright staining for fibrin within endothelial cells, as well as occasional deposits along the basement membrane. Gamma globulin was only occasionally demonstrable, generally in the form of irregular deposits along the basement membrane. ß1C was absent and albumin was not seen in glomeruli, except sometimes in the form of droplets within epithelial cells. In biopsies from pregnant patients without toxemia only equivocal staining for fibrin was seen. On the basis of these observations and other evidence discussed, it is proposed that the accumulation of fibrin in glomeruli reflects a prolonged state of intravascular clotting in toxemia and that the arrest in glomeruli of some form of circulating fibrin constitutes the basic pathogenic mechanism of the glomerular damage in this disease.