Following platelet stimulation by agonists, integrin-alpha IIb beta 3 (or glycoprotein IIb-IIIa) is converted to an activated state that can bind soluble fibrinogen and mediate platelet aggregation. However, little is known about modulation of alpha IIb beta 3 in cell lines. In the present study, we show that agonist stimulation modulates alpha IIb beta 3-dependent adhesive properties of a human erythroleukemic (HEL) cell line. Brief treatment with phorbol 12-myristate 13-acetate (PMA) caused a significant increase in HEL cell adhesion to monoclonal antibodies (MoAbs) specific for activated alpha IIb beta 3 (PAC1 or pl- 55). This adhesion was inhibited by blocking MoAbs or peptides specific for alpha IIb beta 3, but not by anti-Fc gamma receptor-specific MoAb. Similarly, PMA enhanced HEL cell adhesion to immobilized fibrinogen by 10-fold. However, the activation-dependent ligands in solution (ie, PAC1, pl-55, or fibrinogen) did not inhibit the enhanced HEL cell adhesion to immobilized MoAbs PAC1 or pl-55 after PMA treatment. Thus, PMA may increase alpha IIb beta 3-dependent adhesion to immobilized activation-dependent antibodies and fibrinogen by increasing the local concentration of alpha IIb beta 3 to participate in low-affinity interactions, resulting in an increased avidity, changing the affinity state of alpha IIb beta 3, or both.