The results reported in this paper show a highly significant fall in cloning efficiency in unirradiated normal and malignant epithelial cell lines receiving medium from irradiated cultures. Medium irradiated in the absence of cells had no effect nor did irradiated medium from a fibroblast line, but irradiated medium from epithelial cells had an extremely toxic effect on unirradiated fibroblasts. Cells from four different cell lines were seeded, using standard techniques, 6 h prior to irradiation with cobalt 60 (Co60). At various times ranging from 1-60 h after irradiation, medium was removed, passed through a 0.22 mu filter and then used to replace the medium from parallel cultures seeded at cloning densities which had not been irradiated. The effect produced by the epithelial cell cultures was dependent on the cell number present at the time of irradiation, suggesting that a cell-derived factor is involved. The effect could be observed using medium taken from irradiated cultures as soon as 30 min/post irradiation. Examination of unirradiated cultures 48 h after receiving irradiated medium revealed the presence of high numbers of apoptotic bodies and other morphological evidence suggesting apoptosis may be a prominent mechanism of cell death responsible for the reduced cloning efficiency.