The results of a series of mouse-skin paintings are given for fractions prepared by two schemes designed to concentrate the polycyclic aromatic hydrocarbons (PAH) and their heterocyclic analogues (HETC) present in cigarette smoke condensate into single fractions. It is demonstrated that, for each group, a single index of tumour response, the "Weibull risk parameter" (WRP), can be calculated which, considered in conjunction with two other parameters common to all the groups, adequately describes the pattern of tumour incidence in that group. These indices can be used to calculate for each fraction a further statistic, the "tumorigenic ratio" (TR), which conveniently measures the activity of the fraction relative to whole-smoke condensate on a weight-for-weight basis. From the analyses it is shown that the separation processes can successfully concentrate all types of mouse-skin carcinogenic material, irrespective of the type of condensate used, and that a combination of processes prepares an active concentrate representing 2% by weight of the original condensate.