Fibroblast-pneumonocyte factor, produced by the fetal lung fibroblast in response to glucocorticoids, was partially purified by column chromatography on Sephadex G-75. The resulting preparation showed two major and two minor bands on sodium dodecylsulfate-polyacrylamide gel electrophoresis. When fetal rats were injected on day 17 of gestation with 1 microgram of this material, they showed on day 20 biochemical evidence of accelerated lung maturation as compared to littermate controls. There were no differences between the two groups in body weights, organ weights, or circulating corticosteroid levels.