Lymphocytes from eleven patients with aplastic anæmia were cultured with various agents which interact with D.N.A., and the proliferative responses to phytohæmagglutinin were measured. Lymphocytes from seven of the eleven patients were unduly sensitive to bleomycin, an agent causing strand breaks in D.N.A. The findings suggest that D.N.A. in these seven patients was abnormal, possibly as a result of an abnormality of D.N.A. repair. It is suggested that in aplastic anæmia D.N.A. damage in stem cells may lead to a failure of proliferation.