Yeast cultures progressing from the exponential to the stationary phase of growth showed changes in cell sensitivity to physical agents such as UV light, heat shock at 52 degrees C and the chemical mutagens ethyl methane sulphonate, nitrous acid and mitomycin C. Exponential phree chemicals. The increased resistance of exponential phase cells to UV light was shown to be dependent upon the functional integrity of the RAD50 gene. Treatment of growing yeast cultures with radioactively labelled ethyl methane sulphonate indicated the preferential uptake of radioactivity during the sensitive exponential stage of growth. The results indicated that the differential uptake of the chemical mutagens was responsible for at least a fraction of the variations in cell sensitivity observed in yeast cultures at different phases of growth.