Blood, urine, blood pressure and subjective symptoms of 32 workers in a B H C plant were examined in order to investigate B H C hazards. Slight anaemia, leukopenia and eosinophilia were found in about 40% of the examinees and number of items of subjective symptom increased in proportion to the length of service. However, it is natural to think that these findings were solely due to B H C poisoning, because such changes in the blood are found in common among workers of any other plants and many parasite egg-holders were found among the examinees. Findings on three workers who were suddenly attacked with loss of consciousness, weakness of muscle and convulsion were reported here. Besides these chief symptoms, leukocytosis and neutrophilia were observed in the blood. These cases were diagnosed as B H C poisoning.