An analysis and interpretation is presented of published data concerning the dependence of radiobiological effectiveness on the radiation quality of photons, neutrons and heavy ions for the induction of these two effects in different types of mammalian cell. The results of this analysis suggest that chromosome aberrations observable at mitosis show a stronger dependence on YF or LET infinity than cell inactivation. At high YF, observable abberrations provide a major contribution to cell reproductive death induced by small doses. At low YF the effectiveness of small doses for cell death depends mainly on another type of damage, possibly also induced in the chromosomes, but not observable at mitosis. This type of damage depends less of YF or LET infinity than observable aberrations. The implications of these differences in damage in relation to radiation quality for the extrapolation of data on other types of damage to small doses of interest in radiation protection are discussed in relation to maximum r.b.e values observed.