It has recently been shown that, when irradiated, human epithelial cells produce a factor or signal in the culture medium that can reduce the clonogenic survival of unirradiated cells. The mechanism is unknown, as is the nature of the signal or substance. In this paper, we show that the medium from these irradiated cells is able to induce delayed effects in the progeny of some cell types that survive the initial exposure to the medium. The initial clonogenic survival of normal human keratinocytes exposed to medium from irradiated parallel cultures is reduced by approximately 40%. If the surviving keratinocytes (60%) are grown to confluence and replated for clonogenic assay, than they still show a reduced plating efficiency, this time of approximately 20% less than the parent line. Similarly treated normal human fibroblasts showed no delayed effects either from a direct dose or from receipt of irradiated medium. The progeny of directly irradiated tumourigenic cell lines have previously been shown to have better clonogenic survival (by a factor of at least two) than unirradiated parallel cultures, and this effect was also found, although it varied depending on the cell line used, when the distant progeny of PC-3 cells or SW48 colon carcinoma cells that received medium only from irradiated progenitors were assayed for expression of delayed lethal mutations. These data suggest that the signal/factor produced in medium by irradiated cells is able to induce genomic instability-type effects in distant progeny.