The leukaemic cells in a 23-year-old man were small to medium-sized lymphoblasts with no cytoplasmic vacuoles and negative with PAS as well with peroxidase and acid phosphatase staining. Cytogenetic analysis showed -6, +12, -22, +mar (6p::22q), resulting in a trisomy 12 and monosomy of the long arm of chromosome 6. Immunological marker analysis revealed that the majority of the blasts was positive for terminal deoxynucleotidyl transferase (TdT) as well as surface membrane immunoglobulin (SmIg, mu, lambda), although B-ALL are supposed to be negative for TdT. The blasts were also positive for HLA-DR, CD9 (BA-2), CD10 (VIL-A1) and CD24 (BA-1), but negative for the B-cell markers CD20 (B1) and Y29/55. Double immunofluorescence staining confirmed that almost all TdT+ cells were also positive for Sm mu, Sm lambda, HLA-DR and CD10. We thus made a diagnosis of TdT+ B-ALL without Burkitt characteristics. Since we could not detect SmIg+/TdT+ cells in bone marrow samples from adult healthy volunteers and from 10 children with ALL in complete remission, we conclude that TdT+ B-ALL cells may not have a normal counterpart in bone marrow or represent a malignant counterpart of a very rare cell in an intermediate differentiation stage between the pre-B-cell and the early B lymphocyte.