When rat liver and lung microsomes were incubated with crocidolite fibres, a substantial increase in lipid peroxidation was observed. Increasing concentrations of this fibre in the incubation mixture, produced a concomitant induction in microsomal lipid peroxidation. This increase was very rapid as the reaction was almost complete within 10 min. The ability of these fibres to induce this process after a short period of incubation seems to be due to their binding to the microsomes. The iron present in the structure of crocidolite is thought to be responsible for catalysing the peroxidation. The formation of free radicals by asbestos and possible pathological sequelae are discussed.