Serum levels of Russell's viper venom in 30 patients bitten by Russell's viper were measured by an ELISA. In the initial serum samples, which were collected immediately after admission to the hospital (0.5–19 hr after the bite), venom was detected in 24 patients (80%), with levels ranging from 3 to 92 ng/ml. These levels correlated well with the patient's clinical signs. At 6–12 hr after antivenom therapy, the venom levels in most of the serum samples (21 of 24, 87%) had decreased to undetectable levels. In the remaining patients, whose serum venom levels could be detected 36–72 hr after therapy, the levels varied according to the effect of the antivenom therapy and the release of venom from a deposit at the bite site.