Carbostyril derivatives and salts thereof represented by the general formula (1a), including some known compounds, possess activities for inhibiting adhesion of thrombocytes. ##STR1## Z, A, X, R, R.sup.1 and R.sup.2 are defined in the specification. Some carbostyril derivatives having chemical structural formulas similar to those of carbostyril derivatives and salts thereof represented by the general formula (1a) have been known in prior art references, however the above-mentioned pharmacological activities have not been known to the present date.