Novel 1,5-benzothiazepine derivatives of the formula: ##STR1## wherein R.sup.1 and R.sup.4 are:(a) R.sup.1 is lower alkyl or lower alkoxy, and R.sup.4 is lower alkyl, lower alkoxy, fluorine, benzyloxy, hydroxy or lower alkylthio, or(b) R.sup.1 is lower alkyl, and R.sup.4 is hydrogen, or(c) R.sup.1 is hydroxy, and R.sup.4 is lower alkyl, fluorine, hydroxy or lower alkylthio;R.sup.2 is hydrogen, lower alkanoyl or benzoyl; and R.sup.3 is lower alkyl; and a pharmaceutically acceptable acid addition salt thereof are disclosed. Said derivative (I) and its salt have a potent platelet aggregation-inhibiting activity.