Gene mutation in vivo in human T lymphocytes appears to occur preferentially in dividing cells. Individuals with multiple sclerosis (MS) are assumed to have one or more populations of dividing T cells that are being stimulated by autoantigens. Mutant T cell clones from MS patients were isolated and tested for reactivity to myelin basic protein, an antigen that is thought to participate in the induction of the disease. The hypoxanthine guanine phosphoribosyltransferase (hprt) clonal assay was used to determine mutant frequency values in MS patients with chronic progressive disease. Eleven of 258 thioguanine-resistant (hprt^-) T cell clones from five of the six MS patients who were tested proliferated in response to human myelin basic protein without prior in vitro exposure to this antigen. No wild-type clones from these patients, nor any hprt^- or wild-type clones from three healthy individuals responded to myelin basic protein. Thus, T cell clones that react with myelin basic protein can be isolated from the peripheral blood of MS patients.