Operation procedure for animals. A: S group. B: CS group. DMBA was heated to 124°C. A central portion of 3–0 silk suture or a piece of cloth strip was immersed in the melted DMBA; sutures contained approximately 1 mg per centimeter of the carcinogen on average and each cloth strip contained approximately 1.5 mg of the carcinogen on average, as calibrated by a micro-chemical balance. Rats were anesthetized by intraperitoneal injection of chloral hydrate at 50 mg/kg. Then both ovaries were surgically exposed, and a DMBA-coated suture (S group, 80 rats) (A) or a DMBA-coated cloth strip (CS group, 128 rats) (B) was then inserted or packed into both ovaries and closed with the surrounding fatty substance. The arrow referred to the ovary of rat.