Phytohemagglutinin in concentrations of 0 to 800 µg/ml was used to stimulate peripheral blood lymphocytes of normal donors and responses were determined using tritiated thymidine incorporation into DNA after 68 hr incubation. A computer program was used to rank responses at each dose of phytohemagglutinin and to test for a normal distribution by the technique of probit transformation of cumulative frequencies. The procedure was repeated using logarithms, square roots, and cube roots of ranked values. Results indicated that the logarithmic function provided the closest approximation to a normal distribution over the range of the dose-response curve. These results validate the practice of plotting lymphocyte culture data on logarithmic graph paper and indicate that geometric statistics should be used to compare sets of data by methods which assume a normal distribution.