In earlier studies, it has been shown that the retention index (Ir) is a very reproducible measure of gas chromatographic mobility. Statistically comparing Ir values obtained here with those found in references has yielded very satisfying results. This will enable the use of Ir values in the screening phase of analysis for general unknowns using the great number of data offered by references; thus limiting the range of inquiry to no more than 6 or 7 drugs, provided that the unknown is between the scheduled ones. An index of analytical behavior that is an absolutely reproducible number will be necessary for computerizing toxicological analysis.