Simultaneous detection of specific surface markers by immunogold and intracellular peroxidase activity was determined ultrastructurally in normal and leukaemic progenitors of platelets, erythrocytes and granulocytes. A new method of fixation was employed to preserve platelet peroxidase activity. Monoclonal antibodies to platelet glycoproteins labelled exclusively platelet peroxidase (PPO) positive cells, i.e. platelets, megakaryocytes and promegakaryoblasts (PMKB). In acute megakaryoblastic leukaemia, most PMKB possessed both markers while a few PMKB identified by PPO did not bind monoclonal antibodies. This result suggests that PPO appears earlier in maturation than platelet glycoproteins. Although all glycoproteins (GP) displayed fewer sites in PMKB than platelets, GP Ib was often observed in more mature megakaryocytes. Surface (glycophorin A) and intracytoplasmic markers including ferritin, intra-mitrochondrial iron and diffuse peroxidase activity due to haemoglobin of erythroid progenitors, appeared simultaneously. The number of glycophorin A sites increased with maturation. In leukaemia involving PMKB and proerythroblasts, the surface markers were coincident with the localization of peroxidase activity; glycophorin A was always absent from blasts which exhibited PPO activity localized in endoplasmic reticulum. Platelet glycoproteins were never expressed in any other cell lineage. The myeloid surface antigen was present on normal late neutrophilic promyelocytes after the cessation of myeloperoxidase synthesis. In some cases of M1 and M2 AML (FAB classification), labelling was identical to normal cells while in others the antigen appeared earlier than normal. Our findings show that the surface phenotype of blasts from non-lymphoid leukaemia and the intracellular peroxidase activity of a given cell type can be simultaneously demonstrated and analysed by electron microscopy.