In human PMN (polymorphonuclear cells), challenged by P-selectin, the β2-integrin Mac-1 (macrophage antigen-1) promoted the activation of the SRC (cellular homologue of Rous sarcoma virus oncogenic protein) family members HCK (haematopoietic cell kinase) and LYN (an SRC family protein tyrosine kinase) and phosphorylation of a P-110 (110 kDa protein). SRC kinase activity in turn was necessary for macrophage antigen-1-mediated adhesion [Piccardoni, Sideri, Manarini, Piccoli, Martelli, de Gaetano, Cerletti and Evangelista (2001) Blood 98, 108–116]. This suggested that an SRC-dependent outside-in signalling strengthens the β2-integrin interaction with the ligand. To support this hypothesis further, in the present study, we used the monoclonal antibody KIM127 or manganese to lock β2 integrins in a high-affinity state, and homotypic PMN adhesion was analysed to monitor β2-integrin adhesive function. KIM127 or manganese induced PMN homotypic adhesion and P-110 phosphorylation. Both these processes were abolished by blocking antibodies against the common β2 chain, by a combination of antibodies against αL and αM or by inhibitors of SRC activity. Confocal microscopy showed that activation epitopes were expressed by β2 integrins co-localized with patches of F-actin at the adhesion sites. Blockade of SRC kinases or of actin polymerization prevented clustering of activated integrins as well as F-actin accumulation. FACS analysis showed that SRC inhibitors modified neither basal nor manganese-induced KIM127 binding. An SRC-dependent outside-in signalling initiated by β2 integrins was also required for adhesion triggered by interleukin-8. These results confirm the hypothesis that an SRC-dependent outside-in signalling triggered by high affinity and ligand binding is necessary to stabilize β2-integrin-mediated adhesion. Allowing clustering of activated integrins, SRC might link the high-affinity with the high-avidity state. Proline-rich tyrosine kinase-2 appears to be involved in this process.