We have investigated proliferation in bone marrow trephine biopsies from 32 patients with normal or abnormal haemopoiesis, using the monoclonal antibody PC10, which detects proliferating cell nuclear antigen (PCNA), together with immunohistochemical markers of haemopoietic cell lineage. PCNA immunostaining revealed the pattern of proliferation within individual haemopoietic lineages in normal marrow. Two unexpected observations were made: of erythroid cells, only pro-erythroblasts and occasional early normoblasts reacted, and positivity of megakaryocytes was unrelated to nuclear lobulation or CD61 expression. The pathological cases represented conditions in which haemopoiesis is increased (reactive hyperplasia, chronic granulocytic leukaemia, myeloproliferative and myelodysplastic syndromes, megaloblastic anaemia). Increases in the number, and disturbances of the spatial organization, of PCNA-expressing cells were present to a variable extent in all cases. Sheets of PCNA-positive megaloblastoid erythrocytes were frequently found in myelodysplastic and myeloproliferative tissue, associated with marked disturbances in the spatial organization of all haemopoietic lineages. Cases of megaloblastic anaemia due to vitamin B12/folate deficiency also demonstrated greatly increased erythroid PCNA expression, with positivity in some giant metamyelocytes. In addition to reflecting increased proliferation, elevated PCNA expression in some bone marrow pathologies may be due to altered kinetics of the protein induced by disturbances in growth factor production.