A dissociation-enhanced lathanide fluorescence immunoassay (DELFIA) method has been developed for the determination of AR-C68397XX, a novel respiratory therapeutic agent, in human plasma. The method is a 'direct' immunoassay and provides an alternative to the solid phase extraction RIA described in a previous publication, which employs the same specific antiserum. The DELFIA method is suitable for the determination of the analyte at pg ml(-1) concentrations. The non-isotopic label was prepared by complexation of a DTPA derivative of AR-C68397XX with free europium cation (Eu3+). Plasma samples were diluted at least 5-fold prior to analysis to eliminate matrix interference. The calibration range is 10-2000 pg ml(-1), and the LOQ of the method is 50 pg ml(-1) using 50 microl of diluted human plasma sample.