Small beads derivatised with fibronectin or with bovine serum albumin are allowed to attach to BHK cells in suspension at low ratios of beads to cells. In this way populations of cells bearing predominantly one bead per cell can be prepared. We show that the attachment of one bead per cell affects the adhesion and spreading of that cell on substrata, raising adhesion and increasing spreading if the signal molecule is fibronectin, decreasing these quantities if the bead bears BSA. The experiments are conducted in the absence of other sources of exogenous fibronectin and in some cases in the additional absence of endogenous sources. The effects are especially marked if the substratum is adsorbed haemoglobin on which control cells show little attachment or spreading. We further show by interference reflection microscopy and by scanning electron microscopy that the beads are found on the non-adhering side (uppermost or outer) of the cell when fibronectin-bearing beads are used, presumably because fibronectin will not attach to haemoglobin. The increased adhesion and spreading found in such cases must be attributed to an activation produced by the bead, which spreads to other parts of the cell and which activates a fibronectin-independent mode of adhesion.