OBJECTIVE To determine the yields of cell lethality and micronucleus formation measured immediately after irradiation or at delayed times in primary human fibroblasts exposed to X-rays or alpha-particles. METHODS Primary human AG01522B fibroblasts were irradiated with X-rays or alpha-particles and subcultured for up to 30 days post-irradiation. Clonogenic survival and the yield of micronucleated cells in newly divided daughter cells were scored at various times. RESULTS After the initial dose-dependent acute response for both endpoints there was conclusive evidence for the delayed production of de novo genomic instability in the surviving progeny. As well as the production of micronucleated cells, evidence was observed for the production of fibroblasts undergoing apoptosis as measured using conventional morphological scoring, 3'OH end-labelling of DNA fragmentation and externalization of phosphatidyl serine residues to the cell surface. For both radiations, the dose and time dependencies of the apoptotic response were similar to those for micronucleation and loss of clonogenicity. CONCLUSIONS Genomic instability in the progeny of irradiated primary human fibroblasts was observed in the form of micronucleation, apoptosis and delayed reproductive death. An increased effectiveness of alpha-particles versus X-rays was observed for both initial and delayed responses, including apoptosis.