Proximal tubule handling of two human Bence Jones proteins (neutral and acidic BJP) was evaluated using protein A-gold labelling. After 30 min of acute light-chain infusion into 6 rats (alone or in combination with dinitrophenyl-aminopropyl-methylamine [DAMP]), kidney biopsies were processed for immunoelectron microscopy. Antibodies directed at monoclonal lambda light chains, mannose-6-phosphate cation-independent receptor (MPR) and DAMP were used.Labelling density (number of pA-gold particles/µm<sup>2</sup>), expressed as median (25–75 percentiles), differed (p < 0.05) between the two BJP, being 94.5 (32.9–212.5) vs. 19.4 (3.7–45.6) pA-gold/µm<sup>2</sup> in endocytic vacuoles, and 297.3 (207.1–382.1) vs. 83.2 (16.6–197.0) pA-gold/µm<sup>2</sup> in non-vacuolar electrondense endosome-lysosome structures. Labelling density for MPR was 47.7 (22.2–84.6) vs. 4.0 (2.7–6.3) pA-gold/µm<sup>2</sup>. The area of MPR-labelled structures was also different, i.e.: 0.2 (0.1–0.4) vs. 0.9 (0.5–1.8) µm<sup>2</sup>. The endosome-lysosome pH distribution range differed significantly: 6.8 (6.4–7.0) vs. 6.3 (5.8–7.0). There was a significant accumulation of neutral BJP in endocytic structures, an acidification deficit of pre-lysosomes/lysosomes and MPR retention, suggestive of defective receptor recycling with this BJP. Interference with the physiological process of lysosomal acidification may be an important mechanism of higher nephrotoxicity in some BJP.