Using the Raji cell radioimmunoassay of Theofilopoulos et al, soluble immune complex (IC) in sera from 26 patients with systemic lupus erythematosus was measured. The distribution and amount of renal immune deposits were examined. According to the distribution of glomerular immune deposits, patients were divided into four groups. There were 3 patients with the focal segmental mesangial pattern, 8 with the diffuse global mesangial pattern, 11 with the granular pattern and 4 with the membranous pattern. In 2 of the 3 patients with the focal segmental mesangial pattern, serum IC levels were only slightly elevated and mesangial immune deposits were very light. In 7 of the 8 patients with the diffuse global mesangial pattern, serum IC levels were markedly elevated with large amount of mesangial glomerular immune deposits in spite of minimal urinary finding. In patients with this pattern, wide deviation of serum IC levels was observed. In 7 of the 11 patients with the granular pattern, marked elevation of serum IC levels was observed. Three patients with sclerotic glomerurlar changes had relatively low serum IC levels. In 3 of the 4 patients with the membranous pattern, serum IC levels were low. Of the 26 patients of this study, 12 having renal interstitial deposits had higher serum IC levels than 14 patients without renal interstitial immune deposition. This difference was statistically significant. There were no relationships between the serum IC levels and the grade of hematuria or proteinuria. The data suggests that the serum IC levels measured by the Raji cell radioimmunoassay may give us some information about the degree of renal immune deposition and become a useful index of lupus nephritis.