The effect of radioprotector cystamine on structural chromosome mutations induced in Crepis capillaris cells by gamma-rays 137Cs is studied. Cystamine at the concentration of 0,1% causes a two-fold decrease in the frequency of structural mutations. In the "vesting" phenomenon of ring chromosomes, under the same experimental conditions, it also leads to a two-fold decrease in the frequency of exchanges, in relation to the frequency of fragment units in the initial stage, by repair of normal chromosome structure. Cystamine provides protection in structural mutagenesis at the moment of realization of primary mutagenic changes into mutations due to enzyme ligazing chromosome fragments.