The monocellate Thai cobra (Naja kaouthia) is a major cause of snake bite mortality and morbidity throughout Thailand, but neither the local nor the systemic effects of its venom are diagnostic. Species diagnosis is important because only monospecific antivenoms are available for treatment in Thailand. We tested the ability of the ELISA technique to detect venom antigen in the sera of 58 acute snake bite cases including 4 fatalities, and venom antibody in 51 patients bitten between 1 month and 19 years previously. N. kaouthia venom antigen was found in 8 of 33 patients with only local envenoming and in 14 of 20 with local plus systemic (neurotoxic) envenoming, but the mean venom concentration was 33 times greater in the latter group. The serum of 1 fatal case contained banded krait (Bungarus fasciatus) but no cobra venom antigen. N. kaouthia venom antibody was present in sera of patients bitten between 1 month and 7 years previously. Antibody was found in 6 of 8 patients who had had local envenoming alone but in only 19 of 41 who had had systemic envemoning treated by antivenom. The titer of antibody declined with an approximate half time of 2–3 years. One patient had a significant titer of B. fasciatus venom antibody. This study confirms the value of ELISA-immunodiagnosis and the predominance of N. kaouthia as a cause of neurotoxic envenoming in the Bang Phli area. However, the attribution of 1 fatal case to B. fasciatus bite suggests that patients with neurotoxic signs should be given B. fasciatus antivenom if they fail to respond to cobra antivenom.