Effects of synthetic peptides corresponding to known cell-binding sites on cell adhesion to the 110-kD polypeptide and 38-kD fragments. K562 cells, expressing constitutively high levels of 5β1 integrin, were used as reference cells to confirm the inhibitory activity of RGD-containing peptides and to determine the optimal blocking concentration. The optimum for these cells was established to be 50 μg/mL of GRGDSP peptides, and raising the concentration to 550 μg/mL still did not inhibit Sc-1/Ci-1 cell binding. Dose-dependent inhibition curves with CS-1 peptides showed a concentration optimum of 150 μg/mL peptide. B peptide and CS-5 were tested at concentrations up to 850 μg/mL.