Lungs were removed from hamsters at gestational day 12 and cultured in BGJb medium under the following conditions: (1) alone, (2) with 5% fetal bovine serum (FBS), (3) with defined additives (with and without vitamin A), and (4) with defined additives but lacking both epidermal growth factor (EGF) and vitamin A. Patterns of cell proliferation were determined by immunochemical labeling for bromodeoxyuridine (BrdU) and airway branching was evaluated in each explant. After 4 days in BGJb alone, an orderly but limited branching pattern occurred, and labeling was greater in the epithelium than in the connective tissue. With FBS a relatively normal branching pattern occurred, and labeling was nearly equivalent in connective tissue and epithelium. With defined additives the connective tissue was heavily labeled in the compact region supporting the distorted bronchioles and in the loose flange at the periphery; removing only vitamin A did not alter these patterns. Removing both vitamin A and EGF caused an extensive but distorted epithelial branching pattern that extended to the periphery of the explant (a flange of loose connective tissue did not form); BrdU labeling was sparse. These results indicate that EGF played a major role in contributing to alterations in the growth and development of fetal hamster lung.