The problems of adjustment for a natural response rate C in probit analysis were fully discussed by Finney. He gave the maximum likelihood method for estimating the parameters of dosage-response relation. It is not unusual, however, that experimenters must compare the two dosage-response curves came from materials having natural response rates C_1 and C_2. In this case, minute modifications for some formulae shown by Finney are necessary to calculate the relative susceptibilities of testing organisms coming from different sources as mentioned above. As a numerical example, the procedure was illustrated by the experimental data comparing the inhibitory concentrations of PCP for the emergence of rice seeds harvested in different seasons.