Antibiological actions were studied on 2-amino-1, 4-naphthoquinone imine-HCl (ANQI), which had been investigated in vitro as a potent inhibitor of DNA synthesis in Ehrlich mouse ascites tumor cells and as an interacting substance to DNA. The results are as follows : 1. Antimicrobial Action : A considerable activity was found against animal and plant microorganisms, especially Xanthomonas oryzae. 2. Acute Mammalian Toxicity : LD50 was 5.45 (4.88-6.09) mg/kg in mice (intraperitoneal injection). Only a little and transient effect was observed on behaviors of mice, and on respiration and blood pressure of dogs. 3. Antitumor Activity : ANQI exerted no therapeutic effect on Ehrlich mouse ascites tumor in vivo, but shortened survival days of tumor-bearing mice in its high doses. 4. Chemical Changes in Mouse Ascitic Fluid : ANQI in part was oxidized in ascitic fluid of tumor-bearing mice to 2-amino-1, 4-naphthoquinone, which had been found to be not inhibitory to nucleic acid synthesis. The ineffectiveness of ANQI on the tumor in vivo was parlty explained from the susceptibility to biological oxidation.