Endotracheal administration of bleomycin to hamsters causes severe pulmonary fibrosis. We have examined the utility of this response as a model for the screening of pulmonary antifibrotic agents. The time course of collagen synthesis after bleomycin administration was examined in neutral salt soluble and insoluble fractions by in vitro incubation of minced lung with [14C] proline. Collagen synthesis increased to approximately 250% above control in both neutral salt soluble and insoluble collagen fractions by day 6 after bleomycin. Noncollagenous protein synthesis was also increased but to a lesser amount. The early rise in collagen synthesis leads to accumulation of collagen that can be biochemically quantitated within 1 week. This time course is advantageous for short-term testing of antifibrotic agents. In the present study, beta-aminopropionitrile, D-penicillamine, and p-aminobenzoic acid were examined. All three agents were found to reduce significantly the accumulation of neutral salt insoluble collagen in bleomycin-treated animals.