The development of mathematical models for determining antagonism, synergy or additivism for drug combinations is becoming increasingly important in medical research and medicine. This paper describes improvements in the Chou and Talalay method which provides for the first time confidence intervals for the combination index. The Monte Carlo technique was used to calculate the confidence intervals and to obtain the new parameters which are needed for the modified interaction diagnosis. This new method can be adapted to current as well as future formulae used to evaluate drug combinations. Programs written in Microsoft and Visual Basic were developed for use on commonly available personal computers which we call Combostat.