Abstract Plasma immunoreactive endothelin-3 (ir-ET-3) concentrations were measured by a sandwich-enzyme immunoassay (sandwich-EIA) for endothelin-3 (ET-3). The assay method consists of two antibodies directed against N-terminal and C-terminal portions of ET-3. It detects as little as 0.1 pg/well of ET-3 without the crossreaction with endothelin-1, endothelin-2 and big ET-3. Plasma ir-ET-3 concentrations were found to be 0.45 ± 0.07 pg/ml (mean ± SD) in healthy volunteers, and were increased in patients undergoing haemodialysis (0.83 ± 0.26 pg/ml, p < 0.001). In reverse-phase HPLC, ir-ET-3 in normal plasma and in plasma of haemodialysis patients was eluted at the position of authentic ET-3, indicating that ir-ET-3 in plasma detected by the EIA was ET-3 itself. These results suggest that circulating ET-3 exists in normal human plasma and that production and/or metabolism of ET-3 may be altered in patients undergoing haemodialysis.