The effects of Cu, Fe, Pb and Zn as well as of a Pb + Zn combination on the total, available and nonsedimentable (NS) activities of lysosomal and peroxisomal enzymes were examined. An activating influence on the total activities of liver acid phosphatase (AP) and cathepsin D was shown for Cu. In the kidney the heavy metals induced changes in the total activity only of catalase. The effect of Cu was inhibiting, while that of Pb and of the Pb + Zn combination was activating. Copper produced an increase of NS protease and AP activities in liver homogenates accompanied by a rapid release of latent AP from liver large-granule fractions. According to these data and to generally accepted criteria for assessment of the integrity of lysosomes, Cu can be regarded as a powerful labilizer of lysosomal membranes. This heavy metal induced such an effect on liver peroxisomes as well, a statement which is based on the enhancement of NS catalase activity. In the kidney, Pb and the Pb + Zn combination were shown to produce a significant lowering of NS catalase activity, indicating a stabilization of peroxisomes.