There is not only a broad variation concerning in vitro shoot forming capacity among various idiotypes of Gerbera jamesonii, but there also exist differences in radiosensitivity measurable by the X-ray dose dependent inhibition of shoot development. Investigations on the existence of a correlative interaction between 'high shoot forming capacity' and 'low radiosensitivity' or vice versa did not result in clear cut findings, because up to four weeks post-irradiation no influence of shoot forming capacity could be measured. On later dates, however, the influence of shoot forming capacity became more and more significant. This led to the decision to use only the number of shoots on the first date of cut-off to determine radiosensitivity and to fix the so-called 'optimum-dose' for in vitro mutation induction experiments.