The presence of monotypism in thick atherosclerotic lesions of black females with G-6-PD mosaicism first reported by the Benditts (1973) has been confirmed in two other laboratories. However, we believe that it is premature to conclude that the finding of monotypism necessarily indicates monoclonal origin of athero sclerotic lesions. We have suggested two alternative explanations for the obser vation of monotypism which we believe must be shown to be invalid before accept ing monoclonal origin as the only plausible way to account for the observed G-6-PD monotypism. One of these two...