The purpose of this study was to determine the performance characteristics of the Cozart microplate EIA for detecting opiates in oral fluid from patients in a drug misuse treatment program. Oral fluid samples were collected using the Cozart RapiScan Collection System from 216 donors who were receiving treatment for their addiction and were monitored for drug abuse. A further 40 oral fluid samples were collected from volunteer donors who were not drug users. The samples were analyzed in the laboratory by using the Cozart microplate EIA for opiates and confirmed using gas chromatography-mass spectrometry (GC-MS). The samples were stored frozen until analysis by GC-MS. The intra-assay precision for the Cozart microplate oral fluid EIA for opiates over 40 assays was 0.43% to 9.13% CV (within assay) and 2.9% to 9.1% CV (within day). A total of 109 samples were positive for various opiates. The Cozart microplate EIA for opiates in oral fluid, using a cut-off of 30 ng/ml morphine equivalents in neat oral fluid, had a sensitivity of 99.1 +/- 2.1% and a specificity of 94.4 +/- 2.2% versus GC-MS. A series of potential adulterants of oral fluid were evaluated and shown not to alter the outcome of the test result.