A chronic oral toxicity study of 6-amidino-2-naphthyl 4-[(4, 5-dihydro-1H-imidazol-2-yl) amino] benzoate dimethanesulfonate (FUT-187), a new protease-inhibiting agent, was carried out using male and female beagle dogs. FUT-187 was orally administered to the dogs at dose levels of 7.5, 15, 30 and 60 mg/kg/day for 52 weeks, followed by 5 weeks' recovery period. Results are summarized as follows: 1. In general conditions, vomiting, salivation and the passage of mucousy stools were observed in dogs given 15 mg/kg/day or more, and diarrhea was observed at 30 mg/kg/day or more. One male given 15 mg/kg/day showed transient pallidity of the oral mucosa, and another male in the same group showed apnea and abdominal breathing. In addition, one male given 30 mg/kg/day was euthanatized due to extreme weakness, as weight loss and pallid oral mucosa, and another male in the same group died after showing acute toxic symptoms such as hyperpnea, tonic convulsion and ataxic gait. 2. Weight gain was slightly suppressed in females given 60 mg/kg/day. No significant changes in food consumption were observed. 3. Hematological examination revealed no statistically significant changes. Decreases in RBC counts, Ht values and Hb concentrations, and increased reticulocyte counts were observed in one male of 15 mg/kg/day group, which also showed pallid oral mucosa, and in one male of the 30 mg/kg/day group, which was euthanatized in a moribund state. 4. Blood biochemistry revealed increased GPT activity in males given 15 and 30 mg/kg/day and females given 60 mg/kg/day, which was accompanied by sporadic increases in GOT, AlP and/or γ-GTP activities. Males given 30 mg/kg/day or more showed decreased total protein. 5. Hepatic function testing (ICG test) showed no statistically significant changes. One female given 60 mg/kg/day showed increased accumulating concentration of ICG. 6. There were no toxicological changes in urinalysis, fecal occult blood, renal function (PSP clearance), ophthalmological and electro-cardiographic examinations. 7. In pathological examination, inflammatory cell infiltration and microgranuloma formation in liver were noted periportally or perivenularly in both sexes given 15 mg/kg/day or more (except for 30 mg/kg/day males). In the some cases, atrophy, degeneration and necrosis of hepatocytes and/or fibrosis around inflammatory cells and microgranuloma were observed. In the spleen, one male given 15 mg/kg/day and one female given 60 mg/kg/day showed increased plasma cells in the red pulp. In the case sacrificed in a moribund condition, findings in the liver and spleen similar to those in surviving cases were detected, but were more severe, and the liver showed diffuse fibrosis. Congestive pulmonary edema was observed in one dead animal. 8. After 5 weeks' recovery period, these changes had generally disappeared, suggesting they were reversible. 9. As mentioned above, no toxic changes were observed in 7.5 mg/kg/day group, although various changes were observed in the 15 mg/kg/day or more groups in both male and female dogs. Accordingly, the non-toxic dose level of FUT-187 was estimated to be 7.5 mg/kg/day.