LIGAND-BINDING assays are used for determination of minute amounts of substances in the bloodstream. Such assays require a receptor that specifically binds the substance of interest. The receptor used is often an antibody^1–5, but antibodies require special handling and a costly production procedure⁵. We have used molecular imprinting, a method for creating selective recognition sites in synthetic polymers^6–8, to prepare polymers that mimic antibody combining sites. Molecular imprints made against theophylline⁹ and diazepam¹⁰ showed strong binding and cross-reactivity profiles similar to those of antibodies. Here we describe a new radiolabelled ligand-binding assay, the molecularly im-printed sorbent assay, which uses antibody mimics. This assay accurately measures drug levels in human serum, with results comparable to those obtained using a well established immunoassay technique. Antibody mimics, which are stable and readily prepared by molecular imprinting, may provide a useful general alternative to antibodies.