CD151 (PETA-3/SFA-1) is a member of the transmembrane 4 superfamily (TM4SF) of cell-surface proteins and is expressed abundantly both on the cell surface and in intracellular membranes by the haemopoietic cell lines M07e, HEL and K562. In the presence of mild detergent (CHAPS), CD151 co-immunoprecipitated with integrin alpha 4 beta 1, alpha 5 beta 1, alpha 6 beta 1 and alpha IIb beta 3. The association of CD151 with alpha 4 beta 1 and alpha 5 beta 1 seemed to be constitutive, as it was not modified by treatment of M07e cells with cytokines that regulate integrin function by 'inside-out' signalling. CD151 also associated with other tetraspans in an apparently cell-type-specific fashion, as defined by its co-precipitation with CD9, CD63 and CD81 from M07e cells, but not from K562 cells, which express similar levels of these proteins. F(ab')2 fragments of monoclonal antibodies (mAbs) against CD151 caused homotypic adhesion of HEL and K562 cells that was dependent on energy and cytoskeletal integrity and was augmented in the presence of RGDS peptides. The adhesion was not blocked by function-inhibiting mAbs against beta 1 or beta 3 integrins, suggesting that cell-cell adhesion was not mediated by the binding of integrin to a cell-associated ligand. Furthermore, mAb CD151 did not affect adhesion of the cells to fibronectin, laminin, collagen or fibrinogen, which are ligands for alpha 4 beta 1, alpha 5 beta 1, alpha 6 beta 1 and alpha IIb beta 3 integrins. Taken together, these results indicate that the ligation of CD151 does not induce the up-regulation of integrin avidity, but might act as a component of integrin signalling complexes.