Dose-effect curves were fitted in optimizing a non-linear cost function by the simplex method. Two kinds of mathematical functions (Gompertz and Von Bertalanffy) and some weighting processes were studied. In each case a statistical analysis was carried out to choose the curve which is best fitted to experimental data. From the best curve, the efficient doses were calculated. This work was illustrated by cytotoxicity studies concerning melphalan and pyrrolidin.