Rho GDP-dissociation inhibitor beta (RhoGDI beta), a regulator of the Rho family of proteins, is expressed abundantly in the hematopoietic cell lineage. During apoptosis of hematopoietic cells, RhoGDI beta is cleaved by caspase-3 at Asp19 and this cleaved form (.19-RhoGDI beta) has been implicated in the apoptotic pathway. To clarify the role of RhoGDI beta in hematopoietic cells, the present study performed immunoblotting and immunofluorescence staining to examine the expression of RhoGDI beta and.19-RhoGDI beta during phorbol 12-myristate 13-acetate (PMA) -stimulated differentiation of human THP-1 monocytic cells to macrophages. During differentiation of the THP-1 cells to macrophages, the expression of RhoGDI beta remained stable; however, the expression of.19-RhoGDI beta increased, particularly in well-spreading, non-apoptotic cells, which differentiated into macrophages. These results suggested that.19-RhoGDI beta has an apoptosis-independent role in the PMA-induced differentiation of THP-1 cells to macrophages.