Oncogenesis and in vitro data (reported elsewhere in detail) are compared on the basis of relative activity by mass and by dimensional fiber parameters. When tumor induction is compared to the number of fibers of various lengths and aspect ratios in the dose in rats to the degree of tumor induction, a degree of difference with the long thin fiber concept of tumorigenesis by mineral fibers is noted. Consistency is re-established, however, when cognizance is taken of the change in the length and aspect ratio that took place during residence in the lung. This change resulted in a severalfold excess for ferroactinolite of all fiber lengths with high aspect ratios, produced as a result of longitudinal splitting of the introduced fibers. The response by mass in the in vitro procedures did not mimic oncogenesis. When mass was so adjusted that there were an equal number of mineral fibers, aspect ratio > 3, for dose for the two minerals, agreement was closer in both the rabbit alveolar macrophage toxicity test and the clonal cytotoxicity assay in Chinese hamster ovary cells. When activity was related to the number of mineral fibers, the same aspect ratio computed to have been contained in the mass dose, agreement with the relative induction of lung tumors was closer. In all cases, erythrocyte lysis was more active in reflecting the number of mineral fibers.