A structure-activity relationship of some derivatives of 2-phenylsubstituted- 3-hydroxyquinolin-4(1H)-one-7-carboxamides was systematically studied using combinatorial solid-phase synthesis and in vitro cytotoxic activity screening on representative cancer lines. The effect of substituent type in position 2 as well as of the carboxamide group was investigated via synthesis of generic libraries constructed with respect to polarity and bulkiness of appropriate substituents. The process of development afforded a set of compounds with significant cytotoxic activity. Subsequently, corresponding 2-phenylsubstituted-3-hydroxyquinolin-4(1H)-one-6-carboxamides and 2-phenylsubstituted-3-hydroxyquinolin-4(1H)-one-8-carboxamides were prepared to evaluate the influence of the carboxamide group position on the resulting biological activity.