Several techniques have been described for the determination of morphine in hair as a method of monitoring past heroin use. However, although some of the techniques [notably radioimmunoassay (RIA)] may appear relatively simple to perform, any results obtained must be interpreted with caution. In this study, hair specimens from four known heroin abusers were sectionally analysed by a specific RIA for morphine. Prior to analysis, all hair sections were cleaned to remove any possible surface contamination. Five different hair digestion procedures were evaluated to determine the most effective method that could be used to liberate morphine from hair. The greatest analytical recovery was obtained by incubation with 1.0 M sodium hydroxide for 18 h at 55 degrees C, neutralization with 1.0 M hydrochloric acid, and pH adjustment with 0.1 M phosphate buffer (pH 7.0). The morphine concentrations detected in the hair specimens ranged from 0.5 to 13.2 ng/mg of hair. It was also found that the use of shorter length segments (e.g. 1 cm length) gave a clearer, more detailed picture of the historic pattern of heroin use in the four subjects studied.