Colony-forming ability was employed in evaluating the susceptibility toin vitrogamma ionizing radiation in human diploid skin fibroblasts (HDF). Twelve pairs of HDF, each composed of fibroblasts from excised keloid lesion and local normal skin tissue as its control, were studied in patients with clinically persistent keloids. Parameters of radiosensitivity, both D0and D10, and growth kinetics were examined. The radiosensitivity in three of the 12 keloids (25%) were demonstrated significantly increased than their counterpart controls, even though no difference in growth kinetics in between. Moreover, a broad range in the radiosensitivity of fibroblast cells was demonstrated and it is suggested that there is a great heterogeneity of cellular response to radiation in HDF.