The chronic toxicity of vitreous fibers is substantially dependent on their biopersistence. Removal of fibers deposited in the respiratory tract is dependent on a combination of physiological clearance processes (like mechanical translocation) and physico-chemical processes like dissolution and leaching. This publication presents data of about 60 different fibers investigated in the biopersistence test which was standardized in the European Union. This test is based on in vivo investigation of biopersistence after intratracheal instillation in rats of a respirable fiber fraction, and it is a basis for the regulatory classification of vitreous fibers. Regression analysis is carried out employing the data of glass fiber compositions and the corresponding results of biopersistence tests (half-times). The study leads to a model that enables prediction of half-times for stone wool fibers as well as for glass wool fibers on the basis of their chemical composition. The aim of this paper was to investigate the stringency of the existing limits for the range of the chemical composition of glass and stone wools in view of the currently available data base. For regulatory purposes, however, this model is currently not sufficient to replace biopersistence tests completely.