Some prompt and delayed effects of single doses of 100-kv. X rays on the growth of the established FL strain of human amnion cells in tissue culture were studied. The parameters of growth measured were cell volume, number of cells per culture, and total volume of cells per culture (culture volume). The effects were dependent on X-ray dose. Small changes in the growth rate were observed after 100 r, but sterilization of the culture required 1500 to 2000 r. Growth in cell number was more sensitive to radiation than growth in cell volume; thus irradiation tended to produce a condition of continued cellular growth accompanied by mitotic inhibition. As a result, the distribution of cell volumes was widened and the entire distribution was shifted toward higher values. Giant cells represented the largest size in the changed distribution. Subcultures from irradiated cultures that had received 100 to 600 r grew out readily. After 1000 r, an exponential growth curve similar to that for unirradiated cultures was obtained after several transfers. The X-ray sensitivity of such “1000 r-recovered” cultures was not strikingly different from that of the unirradiated cultures. The unirradiated cultures, however, had a less pronounced latent period. The recovery of such irradiated cultures involved, inter alia, the elimination of damaged cells by a process that may he considered analogous to natural selection. The bearing of this phenomenon on the prompt and late effects of X rays in vivo is discussed briefly.