Flavonoids are pigments of edible plants. We have recently reported that most flavonoids induce G1 arrest in human cancer cells, and that genistein (an isoflavone) specifically inhibits their cell cycle at G2-M phase. In the present study, apigenin (a flavone) was found to inhibit the proliferation of B104 rat neuronal cells, and flow-cytometric analysis showed that apigenin arrested their cell cycle at G2-M phase. This effect was dose-dependent and reversible when apigenin was removed from the culture medium. Microscopic observation showed that apigenin did not significantly increase the mitotic index compared with the control. Further, apigenin induced morphological differentiation, that is, elongation and arborization of neurites in B104 cells. This is the first report to show that apigenin inhibited the proliferation of malignant tumor cells by G2-M arrest and induced morphological differentiation.