The question of whether cellular radiation sensitivity is independent or related to plating efficiency (PE) is addressed. Three different cell lines, one human and mortal (AG 1522), one rodent and immortal (CHO AA8), and one rodent and mortal (C3H 10T1/2), were investigated. The first two showed a strong correlation between radiation sensitivity and PE, even when, for the mortal cells, the effect of passage number was factored out.