Using the NZB and NZB/NZW F1 (B/W) hybrid mouse as a model for systemic lupus erythematosus, an effort has been made to quantitate: (1) immune complex deposition in the glomeruli by immunofluorescent staining of immunoglobulin, (2) glomerular cellular proliferation by radioautographic measurement of [3H]Tdr incorporation into the glomerular cells in vivo, and (3) glomerular scarring by PAS staining. The relationship between these changes and increasing age has been examined. By radioautography it was observed that dividing glomerular cells were labelled in vivo after injection of [3H]Tdr. This provided a reproducible measure of the proliferative process in the nephritis of B/W mice. In C57B1/6J and CBA/J mice, which have a low incidence of glomerular disease, little change in the amount of glomerular cell proliferation was observed with increasing age. The NZB strain of animals showed a somewhat increased level of proliferation but this did not increase with age. In striking contrast, glomerular cell proliferation in the B/W mice increased rapidly with age. The earliest change observed in the kidney was the deposition of immunofluorescent material in the mesangium and glomerular capillary basement membrane beginning between 3 and 5 months of age and reaching a peak at 9 months. Increase in glomerular cell proliferation began about 2 months after the onset of immune complex deposition but also reached a maximum at 7 months. Glomerular sclerosis was the last change to appear and continued after the other two parameters measured has begun to decline. These data suggest that the deposition of immune complexes in the glomerulus may be an important triggering mechanism for renal cell proliferation and glomerulosclerosis in the B/W mouse. The techniques described would provide a sensitive and reproducible quantitative method for analysing the differential effects of various types of treatment of immune complex nephritis in animals.