Specific heterodimers of alpha and beta integrins are implicated in mediating adhesion and functional activation of mast cells to extracellular matrix (ECM) proteins, determining thus homing, secretion and tissue distribution of these cells. In the present study, we have examined integrin expression and associated morphological features of mast cells adhering to ECM, also depending on cell activation and under the influence of protein kinase C (PKC) inhibitors. Unstimulated and PMA-activated human leukaemic mast cells (HMC-1 line) were allowed to adhere to fibronectin or vitronectin-coated surfaces. Cells were specifically stained for actin, beta(1, )alpha(1)-alpha(6), alpha(v) and alpha(v)beta(5 )integrins and were evaluated by fluorescence microscopy and confocal laser scan microscopy. Spontaneously adhering cells rapidly assumed an oblong shape, with pronounced formation of filopodia, whereas PMA-stimulated cells were round in shape. Clustering of integrins on filopodia and on comma-like shapes at the cell circumference in rounded cells was noted only for alpha(4), alpha(5) and beta(1 )chains in fibronectin-adhering cells, and for alpha(v) and alpha(v)beta(5) chains in vitronectin-adhering cells. On double staining, clustered integrins co-localized with each other and with actin at the cell membrane and along intracellular tension lines of actin filaments. PKC inhibitors affected the shape of cells, but adhesion was maintained. These data provide a morphological correlate to previously reported functional studies, demonstrating clustering of selected integrins during ECM adhesion at the cell membrane. This was associated with alignment of integrins along actin filaments within the cytoplasm, PKC signalling and changes in shape and activation of mast cells.