The acetylcholine receptor (AChR) of muscle is the target of the pathogenic antibodies in the human autoimmune disease myasthenia gravis (MG). For studies on the autoreactive T cells presumed to be responsible, use of intact human autoantigen would be optimal, but it was thought to be prohibitively scarce. However, adsorption to the surface of immunomagnetic particles (Dynabeads) of intact AChR from whole muscle extracts or from affinity-purified preparations, using mouse anti-human AChR Mabs, largely overcomes this problem. Together with antigen presenting cells (APC), this bead-bound AChR has consistently and maximally stimulated an established MG T cell line (previously selected with recombinant human AChR alpha subunit) that recognises the 144-156 region of the human alpha sequence (Ong et al., 1991). For equivalent T cell stimulation, bead-bound AChR was at least 10(3) times more potent than soluble AChR or recombinant alpha subunit, and 10(6) times more potent than peptide 144-156, implying that antigen in this form is targetted very efficiently to APC and thus to T cells. Finally, we have obtained similar results with T cells specific for other antigens suggesting that this method may have wider applications.