A solid-phase radioimmunoassay to determine the binding of antibodies to lipid antigens is described. Polyvinyl chloride microtiter plates were coated with lipid antigens by placing in each well 25--100 microliters of solutions of the antigens in ethanol and evaporation of the solvent. The wells were than washed with 0.3% gelatine solution to remove loosely bound antigens and to saturate the plastic with an unrelevant protein to prevent unspecific binding of antibodies to the plates. About one-third of [3H] phosphatidylcholine adsorbed in this way (out of 1 nmole) was firmly attached to the plastic and was not washed away during a standard assay. The rest, loosely bound lipid, was washed away in the first 5 washings before the primary antibodies were applied. The technique is rapid and convenient and is as sensitive as a conventional solid-phase radioimmunoassay with protein antigens. It requires minute amounts of antigen. Less than 10(-12) moles of lipid antigen can be detected by this technique.