A luminance thresholding procedure was developed to quantify cell attachment of a variety of cell lines to CBD/RGD, a hybrid attachment factor comprising a cellulose binding domain and the fibronectin-like RGD attachment peptide. The technique used local thresholding, median filtering, and opening to separate and count cells on each image. Cell lines exhibited three different patterns of attachment to CBD/RGD, depending on whether it was immobilized on polystyrene or cellulose acetate. Vero, COS, HFF, 3T3, 293, and U373 cells attached well to CBD/RGD immobilized on polystyrene or cellulose acetate. CHO, MRC-5, and HEp-2 cells attached to CBD/RGD immobilized on polystyrene, but not to CBD/RGD immobilized on cellulose acetate. BHK and L cells failed to attach to CBD/RGD immobilized on either polystyrene or cellulose acetate. The attachment of many cell lines to CBD/RGD was comparable with attachment of these cells to fibronectin.