The phosphorylation state of the CD18-chain of beta 2-integrins have been shown not to mediate changes in the avidity of these receptors (i.e., inside-out signaling); however, no alternative functional significance has been proposed. Our study focused on how changes in the phosphorylation state of beta 2-integrin-receptors on HL60-granulocytic cells are related to its intracellular signal transduction properties (i.e., outside-in signaling). Engagement of beta 2-integrins on differentiated HL60 cells induced a transient increase in the cytosolic free Ca2+ concentration and an increased tyrosine phosphorylation of three major protein bands (70, 115, and 140 kDa). These signaling events occurred without any detectable phosphorylation of the CD18-chain. However, a strong phosphorylation of the CD18-chain by preexposure to phorbol myristate acetate (PMA) coincided with an abolishment of both the beta 2-integrin-induced Ca2+ signal and the protein tyrosine phosphorylations. By comparison, none of these effects were exhibited by 4-alpha-PMA, an analogue that does not activate protein kinase C. Thus, phosphorylation of the CD18-chain of beta 2-integrins is not required for outside-in signal transduction by these receptors, but it could constitute an effective mechanism by which the signaling properties of beta 2-integrins can be modulated by exogenous factors and possibly also by intracellular signals induced by other receptors. The fact that both the cytosolic free Ca2+ signal and protein tyrosine phosphorylations were abrogated by PMA suggests an intimate relationship between these two intracellular signals. To explore this possible relationship, we chelated the beta 2-integrin-induced Ca2+ signal with BAPTA. The beta 2-integrin-induced protein tyrosine phosphorylations were blocked by BAPTA but not by abolishment of the Ca2+ signal due to chelation with MAPT or by pretreatment with thapsigargin. These findings and the observation that pretreatment of cells with methyl-2,5-dihydroxycinnamate (a tyrosine kinase inhibitor) blocked the beta 2-integrin- but not the fMet-Leu-Phe-induced Ca2+ signal suggest that beta 2-integrin-induced tyrosine kinase activation occurs prior to and is a prerequisite for the subsequent Ca2+ signal.