The effect of long-term exposure to the organophosphate insecticide Anthio on serum esterases and ruminal microorganisms of male calves was investigated. The daily oral administration of 3, 6 or 12 mg Anthio/kg/d for 90 d caused significant inhibition of serum cholinesterase (10-28%) and carboxylesterase (12-33%) in male calves. Toxic signs characteristic of anticholinesterase poisoning were observed during 25-70 d of exposure to 6 and 12 mg Anthio/kg. The dose of 12 mg Anthio/kg was lethal to 1/5 calves. Total protozoal population was decreased significantly (15-27%) following 6 and 12 mg Anthio/kg, whereas reduction in total bacterial population (18%) was only significant at 12 mg Anthio/kg.