Adrenomedullin (AM) is a potent hypotensive peptide. Plasma contains mature-type AM (m-AM), which is amidated at the carboxy terminus, and an intermediate, AM-Gly. We developed a one-step two-site IRMA specific for determining human m-AM with monoclonal antibodies. The detection limit was 0.5 pmol/L, and the working range (CV <15%) was 1–300 pmol/L. Dilution of plasma samples showed good linearity. The recovery of added AM was 91–118%. The intra- and interassay imprecision values (CVs) were 4.4–8.2% and 5.5–8.3%, respectively. The assay had no cross-reactivity with AM-Gly or other peptides similar to AM. The mean (± SD) plasma human m-AM concentration of 61 healthy subjects was 1.18 ± 0.65 pmol/L. In conclusion, our IRMA makes it possible to specifically measure m-AM, using a small amount of plasma sample (0.2 mL) by a one-step overnight assay without prior extraction. Our simplified method would be suitable for clinical studies on AM, especially when large numbers of samples must be processed.