It was first reported here that verapamil (VP) and electric beam radiation (EBR) were capable of inhibiting, independently or synergically, clonogenic growth in two kinds of K562 cell lines, adriamycin (ADM)-sensitive and ADM-resistant (K562/S and K562/ADM). Results showed that clonogenic rate (CGR) decreased by 3%-99.9% in the presence of dependent dose-ADM (3.8μg/ml) in K562/ADM cell lines, while treated with 0.5μM-61μM of VP. VP was capable of potentiating radiosensitivity in K562/S and K562/ADM cell lines, whether before or after exposure of them to electric beam radiation, and significantly reduced CGR in these kinds of cell lines (P<0.01).