Detailed chromosome analysis failed to reveal any evidence for spontaneous chromosome instability in lymphocytes from patients with multiple endocrine neoplasia, type 2 (MEN-2), whereas, with one exception, lymphocytes from MEN-2 patients were significantly more sensitive to chromosome damage by bleomycin and, to a lesser extent, MNNG than those from controls. The exceptional case suggests possible genetic heterogeneity in MEN-2.