We examined the expression of CD10 and G-CSF receptor (G-CSFR) on the lymphoid population of mononuclear cells obtained from bone marrow (BM) using two-colour analysis. In the BM of children with ALL in remission, the CD10+ population was significantly increased (20.6 +/- 5.1% compared with that of controls (2.5 +/- 0.5%). More than half (61.3 +/- 2.9%) of the CD10+ cells co-expressed G-CSFR, but not CD13. These results indicate G-CSFR+ B-cell precursors are markedly increased in BM of ALL in remission, suggesting the probable involvement of G-CSF in the human early B-cell ontogeny.