Monoclonal antibodies against L-carnitine have been produced and characterized. These antibodies have been found to specifically bind L-carnitine and, with different affinities, other carnitine-related compounds. No binding was observed with choline or acetylcholine. These antibodies have been used to measure L-carnitine in biological samples and serum. Data obtained demonstrate that, in biological samples, by using radiolabelled carnitine, it is possible quickly to detect small amounts of carnitine. The high specificity of the test is clearly demonstrated.