The gene encoding the superantigen Mycoplasma arthritidis T-cell mitogen (MAM) was molecularly cloned. The mam gene was modified by site-directed mutagenesis to change UGA codons, which in mycoplasmas are read as tryptophan codons instead of universal stop codons, to UGG codons such that the gene could be expressed in standard expression systems. Recombinant MAM, including extra amino acid residues at the N- and C-termini, were expressed and discovered to be biologically active. Certain amino acid substitutions in active regions of the protein also yield biologically active MAM proteins. A method of diagnosing rheumatoid arthritis using recombinant MAM protein in an ELISA assay is disclosed.