Macrophage-like development of myeloid leukemia cells which can be induced by agents such as phorbol esters (TPA) is accompanied by integrin expression and cell adhesion. Thus, in differentiating myeloid leukemia cells CD11b is predominantly expressed which can associate with CD18 to form the functional heterodimeric integrin Mac-1. To elucidate the role of cell adhesion during macrophage-like differentiation, we transfected human U937 myeloid leukemia cells with a vector containing the CD11b gene in antisense orientation. Expression of the CD11b antisense gene in stably transfected U937 cells (as-CD11b cells) resulted in an attenuated response to TPA. As-CD11b cells demonstrated poor adhesion to solid substrate upon TPA treatment in contrast to U937 control cells. Constitutive expression of c-myc in as-CD11b transfectants was higher than in control cells and failed to be repressed by TPA treatment. Moreover, unlike control cells, antisense transfectants failed to induce expression of early response genes such as c-jun and the redox factor ref-1 upon TPA stimulation. Consequently, the induction of monocytic differentiation markers such as the activity of alpha-naphthyl acetate esterase, the capacity to reduce nitroblue tetrazolium and the expression of the vimentin gene was much lower in antisense transfectants than in control U937 cells. According to the failure to undergo a monocytic differentiation program, TPA treatment of as-CD11b cells resulted in a progressively increasing amount of apoptotic cells whereas the differentiated population of U937 control cells remained alive. Taken together, these data suggest that the integrin-mediated (particularly CD11b-mediated) adhesion of myeloid leukemia cells in the course of induced monocytic differentiation is crucial for cell attachment, development of a monocytic phenotype and subsequent survival.