Sesquicillin, isolated from fungal fermentation broth, strongly induced G1 phase arrest in human breast cancer cells. During G1 phase arrest, the expression level of cyclin D1, cyclin A, and cyclin E was decreased, and the expression of CDK (cyclin-dependent-kinase) inhibitor, protein p21(Waf1/Cip1), was increased in a time-dependent manner in a breast cancer cell MCF-7. Interestingly, the G1 phase arrest induced by sesquicillin also occurred independently of the tumor suppressor protein, p53. Sesquicillin inhibits the proliferation of MCF-7 via G1 phase arrest in association with the induction of CDK inhibitor protein, p21(Waf1/Cip1), and the reduction of G1 phase related-cyclin proteins.