Pertussis toxin (PTX) was thought to bind Mac-1 integrin receptor (CD11b/CD18) on TGF-beta1/D3-primed U937 cells and induced cellular adhesion to serum-coated plate. The present study was to investigate the signal transduction pathway utilized by PTX to initiate myeloid cell adhesion in serum. Immunoblotting study showed that PTX induced tyrosine phosphorylation of two cytoplasmic proteins of 150 kDa and 90 kDa in TGF-beta1/D3-primed U937 cells in a time-dependent manner. In addition, PTX-induced myelomonocytic cell adhesion was abolished in the presence of genistein (100 microM), a specific tyrosine kinase inhibitor. 2LPM19c (2 microg/ml), a mouse monoclonal antibody against the CD11b subunit of Mac-1 integrin, or ethylenediamine tetraacetic acid (EDTA, 5 mM) prevented PTX-mediated U937 cell adhesion. On the other hand, nifedipine (1 microM), a calcium channel blocker, significantly reduced PTX-induced U937 cell adhesion. Taken together, it is suggested that binding of PTX to Mac-1 integrin receptor on primed U937 cells triggers protein tyrosine phosphorylation and, to a lesser extent, Ca(+2) influx, which eventually lead to monocytic cell adhesion in serum.