A lignin-related cone extract of pine (Pinus parviflora Sieb et Zucc) (FrVI) or a synthetic lignin (DHP-FA) (175 micrograms/0.1 ml 0.9% NaCl solution) was injected intravenously to SHN mice bearing spontaneous mammary tumours three cycles each with consecutive 3 days of treatment and 4 days of interruption. Activities of both thymidylate synthetase (TS) and thymidine kinase (TK), i.e., DNA synthesizing enzymes in de novo and salvage pathways of pyrimidine metabolism, respectively, were apparently decreased in mammary tumours of FrVI-treated mice compared to those of the control mice bearing tumours without treatment. While the percent change of mammary tumour size during the experiment differed little among groups, both FrVI and DHP-FA prevented tumours from ulceration. Furthermore, the development and growth of preneoplastic mammary hyperplastic alveolar nodules were decreased by the treatments of both agents. They showed no toxicity. All results suggest that these lignin-related compounds, especially FrVI, may be useful as chemopreventive agents, with some improvement of administration method, and/or for employment in combination with any other agents.