In recent years isoniazid has been used as an antituberculous chemophylactic agent. Severe adverse reactions have been reported following its extensive treatment. In addition to hepatic and neurologic disturbances, hematologic alterations have also been reported. Present study was conducted to determine the effect of 0.06 mg isoniazid on the lacertilian packed cell volume. It was 14.0 per cent on day 5, 18.0 and 19.8 per cent on day 10 and day 15 respectively, whereas, it was 24.0, 24.2 and 24.6 per cent in controls on day 5, 10 and day 15 respectively.