We have compared CD10 antigen expression in normal fetal bone marrow with that of B-lineage acute lymphoblastic leukemia (ALL). Both quantitative indirect immunofluoresence (QIFI) and direct immunofluorescence (IF) tests with Quantum beads were used to convert median fluorescence intensity (MFI) values into numbers of antigen molecules expressed per cell (AgE). Lymphoid precursors in the fetal marrow and liver expressed 3–12.5 × 103 CD10 molecules/cell with an upper limit of 5 × 104/cell (MaxAgE). The median CD10 AgE in the different cases of acute B-lineage ALL were variable and ranged from undetectable to very high values (> 1.8 × 105). In 24 of the 72 cases (33%) tested with QIFI the median CD10 AgE was above the highest values seen in normal samples (> 5 × 104/cell). An additional 23.6% of cases had higher median values than the normal median CD10 AgE. Next, CD10 antigen was quantitated in 78 cases during the routine multiparameter analysis of B-lineage leukemia using CD10/class II/CD34 3-color IF test or CD10/TdT 2-color IF test. The aberrant overexpression was confirmed in 43.6% of ALL cases. The CD10bright display suggested ALL diagnosis even when few cells were available for study, e.g., in early relapse and in ALL masquerading as aplastic anemia. The levels of CD10 expression were maintained in relapse. In addition, different CD10 levels were associated with the various chromosomal alterations: high CD10 levels (> 3 × 104/cell) with hyperdiploidy, low CD10 levels (1.8–4 × 103/cell) with the t(1;19), and undetectable levels (< 1.2 × 103/cell) with the t(4;11) translocations. These findings show that while all these diseases are part of the precursor B-cell spectrum, the CD10 changes are linked to disease-associated alterations instead of truly reflecting the features of subtypes of normal B-cell precursors. The quantitative CD10 assessment should therefore be part of the routine flow cytometric assessment in ALL, and further careful studies are warranted during the regeneration phase following chemotherapy. High CD10 expression alone during strong bone marrow regeneration may not be leukemia specific and both the multiparameter analysis described above and parallel studies for gene rearrangements will be necessary to establish the relative values of these assays in detecting minimal disease amidst regenerating marrow. © 1994 Wiley-Liss, Inc.