Plasma beta-endorphin-like radioimmunoreactivity (BEND) was measured in opiate addicts and controls with an antisera which cross reacted with lipotropin but not with other endogenous opiate peptides. 43 stable male methadone maintained patients had mean BEND of 18 +/- 11 pg/ml. No relationship was found between BEND, and time of day, dose or time since last methadone. Controls had 22 +/- 8pg/ml. 9 hospitalized freshly detoxified male opiate addicts, 7-23 days after last opioid, had BEND of 32 +/- 11 pg/ml. These same took 7 days of 50 mg oral naltrexone (NTX) daily, and repeat BEND was 41 +/- 13 pg/ml, rising in all instances compared to baseline. Ex-addicts after detoxification had high BEND which rose further during a short course of NTX. Methadone maintained patients had normal BEND. These data are consistent with previous animal and human reports of high plasma BEND level with NTX or detoxification.