A butenolide compound (1E,3E,5E,7E)-5-hydroxy-4-(8-phenyl-1,3,5,7- octatetraenyl)-2(5H)-furanone (KNK-41), was shown to have strong anti-tumor activity. KNK-41 inhibited the proliferation of various kinds of human malignant tumor cells, such as HeLa (cervical carcinoma), HGC-27 (gastric cancer), PANC-1 (pancreatic cancer) and GOTO (neuroblastoma). Flow cytometric analysis indicated that KNK-41 caused an arrest in G0/G1 phase of the cell cycle. However, it scarcely affected DNA synthesis and the level of c-myc mRNA. These results suggest that the growth-inhibitory effect of KNK-41 is the result of G0/G1 arrest and not of the suppression of DNA synthesis and/or c-myc expression.