Ki-67 is a commercially available monoclonal antibody that reacts with a nuclear antigen detectable in proliferating cells only. Since its first description, it has been widely used as a "universal" proliferation marker and few groups have questioned the validity of the initially described reactivity, although this was tested only on very restricted experimental models. We wanted to check its reactivity on normal bone marrow (BM) samples using a multiparameter flow cytometric analysis. Although we were able to reproduce the findings of Ki-67 positivity on cultured and stimulated cells, we could not detect any convincing Ki-67 positivity on nuclei of normal BM samples. These samples all had a noticeable proliferating compartment as evidenced by their DNA content. These data are in contrast with the data we obtained starting from stressed marrows and marrows cultured in the presence of hematopoietic growth factors, where we found a marked Ki-67 positivity. This discrepancy suggests that bone marrow cells, growing and proliferating under steady-state conditions and guided by natural control mechanisms, may lose their Ki-67 expression upon exiting the progenitor compartment and entering the differentiating compartment.