To investigate the direct toxicity of human Bence Jones protein (BJP), individual nephrons of male Sprague-Dawley rats were perfused in vivo at 20 nl/min with an artificial tubule fluid (ATF) that contained no protein, a human kappa BJP (5 g/dl), or bovine serum albumin (5 g/dl), and proximal convoluted tubule function and morphology were examined. Perfusion with BJP perfusate for ≤5 minutes produced no changes (P = NS) in absorption of water, J(v), (1.09 ± 0.20 vs. 1.50 ± 0.25 nl/min/mm), chloride, J(Cl), (95 ± 47 vs. 123 ± 41 pEq/min/mm), and glucose, J(G), (39 ± 3 vs. 40 ± 5 pmol/min/mm) compared to perfusions with only ATF. However, perfusion for at least 20 minutes with the same BJP perfusate produced decreased (P<0.025) in J(v) (0.58 ± 0.12 vs. 1.15 ± 0.14 nl/min/mm) and J(G) (27 ± 3 vs. 38 ± 3 pmol/min/mm) compared to perfusions with ATF alone; the decrease in J(Cl) (64 ± 47 vs. 119 ± 27 pEq/min/mm) did not reach statistical significance. Perfusion for 20 minutes with ATF containing albumin resulted in no changes in J(v) (1.22 ± 0.21 vs. 1.15 ± 0.14 nl/min/mm), J(Cl) (207 ± 29 vs. 119 ± 27 pEq/min/mm), and J(G) (31 ± 1 vs. 38 ± 3 pmol/min/mm), when compared to the ATF perfusions. Immunocytchemistry, immunofluorescence and immunoelectron microscopy of the BJP-perfused tubules demonstrated the kappa light-chain protein in endosomes and activated lysosomes. In addition, cellular desquamation and fragmentation, prominent cytoplasmic vacuolation, and focal loss of the microvillus border were found in the BJP-perfused tubules, but not in the albumin-perfused tubules. In conclusion, these functional and morphologic data show that a human kappa light-chain is toxic to the proximal convoluted tubule of the rat. This toxicity occurred in a time-dependent fashion when the lysosomal system was markedly activated. Direct damage of the tubule epithelium by BJP's may be involved in the development of the tubulointerstitial nephropathy associated with multiple myeloma.