Immunostaining with NJ-1 monoclonal antibody (MoAb) revealed that NJ-1 is expressed on megakaryocytes (MKs). NJ-1-positive and lineage-negative progenitor cells have a higher potency to proliferate and differentiate into MKs. MKs were divided into NJ-1 (+)MKs and NJ-1(-)MKs. NJ-1 (+)MKs are immature MKs because of their low potential to generate proplatelets. When cultured CD41-positive MK cells were analyzed with RTPCR, we found that the expression of NJ-1 is down-regulated. NJ-1 (+)MKs have a high adherent potential to endothelial cells comparing with NJ-1 (-)MKs, and this binding ability could be inhibited by the NJ-1-Fc fusion protein. We hypothesize that NJ-1 +MKs are immature MKs and the NJ-1 molecule is involved in MK adhesion to endothelial cells. (C) 2002 Elsevier Science (USA).