Masugi nephritis was studied in the male nude and their normal littermate BALB/c mice. In the second phase, severe histological changes consisting of numerous thrombi, thickening of the capillary walls, segmental capillary necrosis, tubular atrophy and numerous casts in the tubular lumen were found in the nephritic nude mice. By electron microscopy, dense deposits were found in the subendothelial space, together with massive fibrin deposition in the capillary lumina. Degeneration and disappearance of endothelial cells could also be seen. On the other hand, the nephritic BALB/c mice showed definite hypercellularity of the glomeruli but less exudative changes. In the second phase, granular deposition of mouse IgG was seen in the mesangium of the nephritic nude mice. In contrast, in the nephritic BALB/c mice, mouse IgG was localized in a linear fashion along the capillary walls. In the plaque forming assay, no indirect plaques against rabbit gamma‐globulin were developed in the nephritic nude mic