A novel monoclonal antibody BI-3C5 recognises myeloblasts and non-B non-T lymphoblasts in acute leukaemias and CGL blast crises, and reacts with immature cells in normal bone marrow.

A MCA raised against the human acute myelogenous leukaemia cell line KG1 reacted with only KG1 among 26 haematopoietic cell lines covering the major lineages. It reacted with early myeloid (M1/2), 1 of 2 acute myelomonocytic (M4) and most non-B non-T leukaemias, including blast crises of CGL. Among M1-AML cells, both MPO+ and MPO- blasts were BI-3C5+. Blasts in 3 Tdt+ M1-AMLs were simultaneously BI-3C5+. BI-3C5 reacted with 4% cells in normal BM, many of which were histologically recognisable as myeloid precursors. 8-15% of BI-3C5+ cells in BM were simultaneously Tdt+, and all were weakly Ia antigen+. BI-3C5 was unreactive with all peripheral leucocytes, with M3 and M5 AMLs, with lymphoid and myeloid leukaemias of "mature" phenotype (T-ALL, B-ALL, CLL, CGL) and with non-haematopoietic cell lines. BI-3C5 precipitated a 120K moiety from 125I-labelled KG1 membranes. It was not blocked by J5 anti-cALLA. The potential use of BI-3C5 in the classification of acute leukaemias is discussed.