Computerized data analysis methods were applied to a selected subset of the results generated by the International Program on Chemical Safety Collaborative Study on In Vitro Assays. The rationale of this study was the analysis of test performances and the comparison with the conclusions of a similar analysis previously performed on the results of the First International Study. The multi-variate data analysis techniques used in the present work proved to be particularly suitable for looking at the complex field of comparative mutagenesis in a logical and systematic fashion, and generated a clear classification of the assays on the basis of their experimental performances. This classification proved not to be consistent with the traditional categories that group tests according to genetic end-point and phylogenetic position, and afforded classes of assays distinct for their sensitivity and specificity in respect to carcinogens. In particular, two systems (Saccharomyces cerevisiae XV185-14C mutation and Syrian Hamster Embryo cells transformation assays) on the basis of the published results, seemed to show a considerable sensitivity in predicting carcinogenicity, although they do not show high specificity.