Cozart Microplate EIA assays are widely used in forensic laboratories worldwide for the analysis of drugs in whole blood and hair. The same properties of high sensitivity, low cutoffs, and cross-reactivity with parent drugs that makes these assays useful for whole blood and hair analysis are also advantageous for the analysis of drugs in oral fluid. As a substitute for heroin use by patients in drug treatment programs, methadone is used as a means of transition into abstinence. The detection of methadone and EDDP, its main metabolite, in oral fluid is a means of monitoring compliance in methadone treatment programs. The current study tested whether the Cozart Microplate EIA assays can be effective in detecting methadone and EDDP in saliva. Using the Cozart RapiScan Collection system, oral fluid specimens were obtained from 198 donors who were receiving treatment for their addiction while being monitored for drug misuse. Oral fluid specimens were also collected from 40 volunteer donors who were not drug users. The specimens were analyzed by EIA and then analyzed for methadone and EDDP by gas chromatography-mass spectrometry (GC-MS). A total of 103 samples were confirmed positive for methadone. Using a cutoff of 30 ng/mL in diluted oral fluid for d-methadone, the Cozart Microplate EIA had a sensitivity of 91.3 percent plus or minus 2.8 percent and a specificity of 100 percent plus or minus 1.0 percent compared with GC-MS. It is therefore a sensitive measure for monitoring compliance in methadone treatment programs. 3 tables, 1 figure, and 20 references