The proliferative capacity of precursor cells in bone marrow and peripheral blood of 19 patients with acute lymphoblastic leukaemia (ALL) at diagnosis was studied and results were compared with the immunophenotype of the leukaemic population. Bone marrow proliferative capacity in these patients was strongly diminished, low or absent, independent of the immunophenotype, compared with control values (p<0.0002). In contrast, the growth pattern in peripheral blood cultures from the same patients varied widely according to the subtype of ALL: whereas in patients with undifferentiated ALL [TdT+, HLA‐DR+, CD19+ or−, CD10−, (n=4) or CD7+, CD5+, CD1−, CD4− and CD8− (n=1)] PB had strongly reduced proliferative capacity compared with control (p < 0.05), there was excess growth of normal neutrophil and erythroid colonies in BP cultures from patients with a more mature immunophenotype of either B‐[CD10+, (n=11)] or T [CD1+, CD4+ and/or CD8+, (n=3)] phenotype. This phenomenon was only seen in patients who had circulating lymphoblasts: If their number was low, growth was so prolific that single colonies could not be identified. In the presence of a high blast count, colony growth was less prolific ‐ probably due to a “dilution” effect ‐ but still higher than normal (p<0.05). We conclude that, in relatively mature ALL of the B‐ and the T‐cell line, the presence of circulating lymphoblasts is associated with increased PB proliferative capacity.