An iteration program is reported by which it will be possible to determine the constants k1 and y0 by the graphical constants tmax and k2. The program is designed for the desk-calculator HP 97. It makes a relatively exact determination of k1 and of y0 possible. By using these constants in the function of Bateman, the latter may be better defined than it would be possible by k1 determined from graphs. Furthermore application of this program saves drawing blood samples within the period of t0--tmax.