Diffuse proliferative glomerulonephritis was induced in the rabbit with injection of goat nephrotoxic serum, and cell dynamics in the glomerulus were quantitatively investigated. The mean total cell number of a normal glomerulus in cross-section was 54.8, consisting of 24.1 endothelial cells, 16.0 podocytes, and 14.3 mesangial cells. In 5 selected nephritic rabbits, the mean total cell number of a glomerulus at the florid stage (7 to 12 experimental days) was 2.4 times as great as the control. It was revealed that the cells consisted of 27.9 endothelial cells, 14.9 podocytes, 21.1 mesangial cells, and 57.8 monocytic cells, indicating that about 76 per cent of the increased cells were of monocytic origin. Mesangial cells showed moderate increase in number (1.5 times) which persisted until 100 days, whereas endothelial cells increased only slightly. The monocytic cells promptly disappeared at 40 days correlating well with resolution of the proliferative change. Although the cellular compositions varied in each case, it was concluded that monocytes were essential in producing proliferative glomerular changes in the reversible type of Masugi nephritis.