A radioimmunoassay (RIA) has been developed for the direct detection of LSD in biological fluids. The radiotracer, (+)-2-[125I]iodo-LSD, allows the use of gamma-counting rather than the liquid scintillation counting currently used for existing 3H radioimmunoassays. The assay is specific for LSD and very closely related compounds. It is inexpensive, sensitive, simple to use and small volumes of samples (50 microliter) can be assayed directly without the need for any time-consuming extraction procedures. The cut-off levels are 1.2 ng/ml in blood and 3.0 ng/ml in urine. The results obtained using the 125I assay described in this work compare very favourably with those obtained using the 3H assay currently used by Home Office Forensic Science Laboratories. The advantages of the assay make it a most appropriate method for the routine screening of LSD in biological samples of forensic interest.