1. Iron, acid phosphatase and N-acetyl-β-glucosaminidase were assayed in liver biopsies from control subjects and patients with primary and secondary haemochromatosis. 2. The activities of the lysosomal enzymes were significantly higher in liver biopsies from patients with iron overload than in those from other patient groups. 3. Lysosomes from the livers of patients with iron overload were strikingly more fragile than those of control subjects as demonstrated by assays of latent and sedimentable N-acetyl-β-glucosaminidase. 4. Lysosomal integrity was essentially normal in biopsies from patients with a wide variety of chronic liver diseases. 5. It is suggested that iron accumulation damages the lysosomal membrane, releasing acid hydrolases into the cytoplasm and thus initiating cell damage.