We investigated the dynamics of expression for morphological transformation, for anchorage-independently growing (Anch-) cells and for mutation at the hypoxanthine guanine phosphoribosyl transferase (HGPRT) locus in X-irradiated Syrian golden hamster embryo (SHE) cells. No Anch- cells were detected with 0-14 days of post-irradiation incubation before selection. No mutants at the HGPRT locus were detected with 0-5 days of post-irradiation incubation before selection. The maximum number of mutants for all doses was found after post-irradiation incubation for 8 days. On the other hand, the highest frequency of morphological transformants for all doses was detected with 0 days of post-irradiation incubation. The frequency of induction of morphological transformants increased with increasing dose. Then morphological transformants abruptly decreased with increasing lengths of post-irradiation incubation and no morphological transformants were detected with 14 days of post-irradiation incubation before selection (less than 10(-4)). A large fraction of morphological transformants (more than 86%) was cloned with feeder cells and expressed more extensive phenotypes of malignant transformation, such as the acquisition of anchorage-independent growth, immortality in vitro and tumorigenicity during further subculturing.