The method here proposed is based on the prevention of photodynamic yeast cell damage by substances possessing radioprotective activity. The photodynamic yeast inactivation was achieved with Toluidine Blue as the photosensitizer and white exciting light. The model radioprotectors tested, namely, WR2721 [S-2(3-amino-propylamino) ethylphosphorothioic acid] and AET [2-aminoethylisothiouronium bromide], were applied at concentrations ranging from 0 to 200 mg/L. Under the conditions of photodynamic damage WR2721 and AFT demonstrated a protective effect as evaluated by the enhancement of the cell survival and colony formation. The protection achieved by AET was more effective. The dependence of the protective effect on the concentration of both agents was linear in the low concentrations range. Experiments with radioprotective preparations of yeast origin demonstrated a similar relationship between the concentration and cell survival. These results indicate that the prevention of the photodynamic yeast cell damage by radio-protectors can find an application as a method for determination of radioprotective effects demonstrated by biotechnologically obtained substances in the course of their production and purification.