Tetramisole produced by ICI, England, and Tetramisole Pharmachim, Bulgaria, were comparatively studied in terms of their acute and subchronic toxicity, and the local and general tolerance and antinematode activity. No differences were found in their acute toxicity (LD50) for albino mice at oral, subcutaneous, and venous application, the index of resorption being 6.3. The same was true so far as the tolerance in calves, sheep, goats, and pigs was concerned. Orally at the rate of 45 mg/kg in sheep and over 30/kg with calves (3, resp., 2 times as high as the therapeutic dose) tetramisole produced nervous and locomotor excitement, tremor, salivation, higher respiration and pulse rate, frequent urination and defecation. Following the tenfold oral administration at intervals, of 3 days at rates of 15 and 45 mg/kg tetramisole Pharmachim did not affect unfavourably the appetite, behaviour, general status, and the clinical and biochemical composition of the blood as well as the structural pattern of the viscera in sheep. Tetramisole Pharmachim was shown to be well tolerated with regard to the conjunctivae at concentrations of up to 5 per cent, the subcutaneous tissues and muscles at conc. of up to 3 per cent, and mucous membranes of the stomach and intestines at conc. of up to 10 per cent.