After gamma irradiation of barley seeds, a comparison has been made between the chlorophyll-mutant frequencies in X1 spikes that had multicellular bud meristems in the seeds at the time of treatment (denoted as pre-formed spikes) and X1 spikes having no recognizable meristems at the time of treatment (denoted as post-formed spikes). The task was attempted in two ways. First, comparisons were made between plant groups with different numbers of tillers. This analysis revealed no significant differences. Secondly, we compared the mutant frequencies of spikes ranked within each X1 plant according to their number of flowers, a ranking which was shown to reflect the embryological status of the spike. In this comparison it was found that (a) the frequency of mutated X1 spikes is larger by a factor of 1·5–2 in pre-formed than in post-formed spikes, (b) the frequency of mutated X2 plants among all recovered X2 plants is the same in pre-formed and post-formed spikes, and (c) the frequency of mutant X3 plants in mutated X1-spike progenies (the segregation frequency) is lower by a factor of almost 2 in pre-formed than in post-formed spikes.