The long interspersed repetitive DNA, L1 or LINE, is a class of mobile genetic elements which can amplify in the cell genome by retroposition, i.e. by a mechanism similar to that of retroviruses. We have shown before that in rat chloroleukaemia cells, maintained in suspension culture in vitro, this element is spontaneously transcriptionally activated at about half of the maximal population density. About 24 h later an explosive amplification of the L1 element is seen in DNA: about 300,000 copies are inserted into apparently random locations in the cell genome, thus creating an outburst of lethal mutations. Dead cells display morphological features typical to programmed death. The present results show that UV light and ionizing radiation induce rapid, premature activation of the L1Rn element during the fast exponential growth of chloroleukaemia cells, and that also this exogenously induced activation is followed by programmed cell death. Transcriptional activation of the L1Rn element can be very strong after the UV exposure: at least 70-fold. Severe hyperthermia, lethal to the cells, does not lead to L1Rn activation (actually a marked suppression is seen) and the mode of phenomic death is necrosis. Some biological implications of the results are discussed.