The binding of clentiazem to human plasma protein and to three purified proteins were measured in vitro. The effect of interaction with various other drugs, which might be used for combined therapy, on the binding of clentiazem was also measured. The percentage of clentiazem bound to human plasma protein was found to be constant (ca. 90%) at the concentration range of 20 ?? 10, 000ng/ml. It would be thought that clentiazem was mainly bound to the albumin fraction of plasma and the binding characteristics of clentiazem to albumin was non-specific. No interference of the various other drugs on the binding of clentiazem to plasma protein were found.