Kappa light chains were demonstrated by immunofluorescence microscopy in the renal basement membranes of 6 of 10 patients with multiple myeloma subjected to kidney biopsy. Nodular glomerulosclerosis was present in 2 of these patients, but the remainder showed only minor light microscopic abnormalities. Deposition of kappa light chains in renal basement membranes may be a frequent cause of kidney disease in multiple myeloma. This deposition may be detected with electron microscopy by the demonstration of granular dense transformation of basement membranes. Occasionally there is systemic vascular deposition of kappa light chains which may be difficult to distinguish from amyloid but lacks a fibrillar character. The tissue effects of light chains in multiple myeloma seem likely to be more extensive than their deposition in renal tubules. There appears to be a greater tendency for kappa light chains to accumulate in renal and systemic basement membranes, but the mechanisms governing this deposition are unknown.