Immunocytochemistry with affinity-purified anti-human cathepsin D was applied to ultrathin frozen sections of human bone marrow megakaryocytes and of blood platelets from peripheral blood. The fixative used was paraformaldehyde (concentration gradient 2----8%). Protein A/colloidal gold (5 and 8) particles were used as second label. Cathepsin D was localized in primary and secondary lysosomes in blood platelets and in primary and secondary lysosomes in megakaryocytes. Primary lysosomes in megakaryocytes were identified by their localization on the trans-side of the Golgi complex and secondary lysosomes by the presence of inclusions. The lysosomes in platelets differed from alpha-granules by being smaller, lacking an electron dense core, and by the presence of a transparent submembrane halo. Platelets undergoing a release reaction after stimulation with thrombin showed cathepsin-D staining in the surface-connecting tubules.