Oncolytic effects of fatty acids in mice and rats. Intrahepatic implants of M114 carcinoma in B6D2F1/J mice were treated by intraperitoneal injection of 30 mg sodium caprylate (octanoate), and implants of Nb2 lymphoma in Nb rats were treated with 300 mg tricaprylin orally. After 4-11 h extensive damage to tumor cells was evident microscopically whereas liver cells were unaffected. Tumors in mice treated once daily from the fourth to eighth day after implantation were obliterated. Subcutaneous implants of hepatoma Nb10L in Nb rats treated transdermally with a caprylic acid preparation underwent similar damage. Fatty acids can cause lysis of tumor cells with little damage to normal tissue in certain situations. This action is not related to mitotic activity and represents a novel mode of antitumor action. 