Hybrid yeast plasmids were constructed, containing the centromere loci CEN3 under the control of two inducible yeast promoters--GAL10 and PHO5. It was shown, that during the induction of transcription from the GAL10 promoter the decrease in mitotic stability of minichromosome is affected both by partial disruption of centromere function by transcription and by influence of galactose on the number of residual cell divisions. In two strains the activity of GAL10 promoter was considerably higher than that of the PHO5 promoter. It is proposed to use the effect of minichromosome destabilization for evaluation of the relative promoter strength.