A radioimmunoassay suitable for screening blood and urine samples for tricyclic antidepressant and some phenothiazine drugs in forensic toxicology is described. The assay displays a remarkably broad spectrum of cross-reactivity that enables therapeutic or subtherapeutic levels of most tricyclic antidepressants and a number of phenothiazines in blood to be detected. The antiserum is commercially available and the radioligand, a radioiodinated conjugate of N-acetyl-L-histidine and nortriptyline, is easily prepared. Blood samples require prior extraction to reduce the background but urine may be assayed directly. 75 microliters of blood or 100 microliter of urine are required. Haemolysis or decomposition of the samples, common anticoagulants and sodium fluoride do not affect the results. Data for 57 compounds are presented.