A scheme for ranking the quantitative activity of chemical carcinogens is described. This activity scheme uses as its base, dose potency measured as TD50, the chronic dose rate that actuarially halves the adjusted percentage of tumor-free animals at the end of the study. The TD50 is converted into an inverse log scale, a decile scale, and then adjusted by weighting factors that describe other parameters of carcinogenic activity. These factors include positive or negative weightings for: the induction of tumors at tissues or organs associated with high historical control tumor incidences; the induction of tumors at multiple sites; the induction of tumors in both sexes of the species; and the induction of tumors in more than one species. These factors were chosen as they represented qualitative descriptions of the general specificity or non-specificity of chemicals with regard to the activity in rodents and have some bearing on the potential activity of chemicals in humans. (Copyright (c) 1990 Elsevier Science Publishers, B.V. (Biomedical Division))