Rheumatoid factors (RF) associated with arthritic joint erosion are only seen transiently, if at all, in nondiseased individuals. Therefore, a tolerance mechanism must exist that prevents pathologic RF B cells from expressing Abs. Surprisingly, it has been shown that pathologic RF B cells are not tolerized by any previously established tolerance mechanism such as deletion, receptor editing, anergy, or prevention of memory establishment. How are pathologic RF cells tolerized? By simulating the RF response with a cellular automaton model immune system, we demonstrate that pathologic RFs can be tolerized by the novel mechanism of "competitive tolerance" with natural, nonpathologic RFs. We then demonstrate that competitive tolerance can be broken when a sequestered pool of expanding B cells are inappropriately subjected to chronic stimulation (as appears to occur in MRL/lpr mice and in patients with rheumatoid arthritis).