Proteinase 3 (P3) is a serine proteinase present in the primary granules of neutrophils. We have investigated the expression of this protein in samples of bone marrow from healthy individuals and patients with different types of leukaemias by using immunocytochemical staining and flow cytometric quantitation. In normal bone marrow the enzyme was found in promyelocytes, myelocytes, metamyelocytes, band forms and polymorphonuclear neutrophils, correlating with the synthesis of neutrophil serine proteinases during myeloid maturation. No staining was found within the lymphoid, erythroid and megakaryocytic lineage. In the leukaemic samples, only those of acute myeloid and chronic myeloid leukaemia patients were labelled with the antiproteinase 3 antibody. Cases of acute lymphoblastic and chronic lymphocytic leukaemia, as well as other malignant lymphomas, were consistently negative, indicating that P3 may be used as a specific marker for the discrimination between myeloid and lymphoid leukaemias. In addition, immunoreactivity of myeloperoxidase (MPO) was investigated and the expression of P3 and MPO correlated with the French-American-British (FAB) classification. P3 was not detected in minimally differentiated M0 and M1 cases but was in predominantly labelled cells of M2 and M3 subtypes plus half of the M4 and one out of six M5 cases but not those of M6. These findings correspond to the differentiation stage in which P3 is expressed and stored in the primary granules. Therefore the enzyme may also be used as an adjunct to the classic morphological and cytochemical methods to elucidate further the stage at which the differentiation arrest of the leukaemic clone has occurred.