A prospective study of the development, intensity and protective ability of the antibody response to Russell's viper bite and its relation to the severity of envenoming was carried out in 15 patients 3 to 116 weeks after the bites using enzyme immunoassay. Retrospective screening in 123 cases revealed antibody in 112/123 (91%) which could be detected from 1 week to 15 years after the bite. A slow peaking (30 weeks) and waning of antibody response was observed in the majority of cases. Development of antibody response was related to the initial venom level, rather than to the delay between bite and antivenom therapy, or the dose of antivenom. Uraemia depressed the IgM response to venom.