The percentage of non-cycling blast cells in children with untreated acute lymphoblastic leukaemia (ALL) was investigated by staining smears for statin, a nuclear protein specifically present in non-growing resting cells. Results were compared with purified normal CD34-positive progenitors. A low fraction of ALL and CD34-positive cells expressed statin (2.9 +/- 3.8% and 2.8 +/- 3.1%, respectively), the growth fraction assessed by staining for the nucleolar antigen p120 was 94% in both ALL and CD34-positive cell samples. From this analysis it can be concluded that the compartment of non-replicating cells in ALL as well as in normal CD34-positive precursor cells collected from peripheral blood is very small and that most cells are cycling.