Lethal effect of 3H-alanine on cells of different ploidy (1n-4n) and radiosensitivity wild type haploid and diploid strains; xrs2 mutant and diploid strains homozygous for this mutation is studied. It is shown that the efficiency of the inactivation per tritium decay depends on ploidy, radiosensitivity and geometric dimension of cells and nuclei. Do for all the studied strains is slightly higher than Do for these strains in case of external beta-irradiation. RBE of beta-ray tritium with respect to beta-ray of 32P is 1.5.