This study examines the structural changes of the lysosomal system of the hemocytes of the zebra mussel Dreissena polymorpha experimentally exposed to lead and zinc. A cytochemical technique which demonstrated acid phosphatase activity as a lysosomal marker was used on blood cell monolayers. The results indicate that the effects of both metals on hemocytic lysosomes were variable and that no marked linear relationship between lysosomal changes and metal concentration and exposure time was observed. Hemocytes of exposed zebra mussels exhibited enlarged and/or more numerous lysosomes. The use of the lysosomal changes in the hemocytes as a biomarker of contaminant exposure is discussed.