The stability of lysosomal membranes was examined during systemic hyperthermia (SH) in Morris 7777 hepatoma-burdened Buffalo rats, since previous studies have shown inhibition of growth patterns of this tumor with SH treatments. Cathepsin D and acid phosphatase were assayed from both ruptured and intact lysosomes from hepatoma and normal hepatocytes. No significant differences were found between results obtained from control animals and those with SH. Cathepsin D hepatoma lysosomes were found to be less stable than those of either normal or tumor-burdened rat hepatocytes. We conclude that lysosomes are relatively stable organelles that do not contribute to the inhibition of growth patterns seen with hyperthermia. © 1982.