Flt3 ligand (FL) is a hematopoietic growth factor, initiating its in tracellular signaling cascade by binding to counterpart receptor and driving receptor dimerization. The native form of soluble FL in vivo is mainly monomeric. In this study, we constructed a rFL-FL fusion protein cDNA by linking two copies of cDNA encoding the soluble domain of FL in tandem and expressed it in Pichia pastoris. On SDS-polyacrylamide gel electrophoresis, the rFL-FL fusion protein showed a molecular weight of 43 kD, agreeing well with the predicted value. The 43 kD protein was further confirmed by Western blot using polyclonal rabbit anti-human FL antibody. The rFL-FL fusion protein exhibited about 10-fold increment in its activity on colony formation of bone marrow progenitor cells. RFL-FL fusion protein also exerted more potent effect than monomeric FL on extending the survival of starving Raji cells.