The presence of collagen in lung is fundamental in normal lung structure and function. Methods have been developed to examine human fetal and adult lung collagen with respect to its composition and synthesis. The second trimester fetal lung has a large number of cells per unit lung mass (36.6 plus or minus 2.7 mug DNA/mg dry wt) and relatively small amounts of collagen (17.0 plus or minus 5.3 mug collagen/mg dry wt). The number of cells per unit lung mass in the adult lung (11.1 plus or minus 3.4 mug DNA/mg dry wt) is 30% of the number of cells in the fetal lung, but the adult has 11 times more collagen (196 plus or minus 25 mug collagen/mg dry wt). The composition of fetal lung collagen can be partially characterized by extraction with salt at neutral pH, acetic acid, or guanidine. The extracted chains, representing 10% of the total lung collagen, chromatograph as alpha1 and alpha2 chains, each with a mol wt of 100,000 and an animo acid composition characteristic for collagen but not specific for lung. Short-term explant cultures of fetal and adult lung synthesize alpha chains which can be isolated by ion-exchange chromatography. These chains, representing 30-40% of the total collagen synthesized by the explants, coelectrophorese with extracted collagen chains on acrylamide gels: they are destroyed by clostridial collagenase and they have a mol wt of 100,000. Although the composition of the collagen synthesized by these explants can be only partially characterized, the rate of synthesis of both collagen and noncollagen protein can be quantitated. In fetal lung, 4.0 plus or minus 1.2% of the amino acids incorporated into protein per hour are incorporated into collagen. In normal adult lung, this percentage (4.2 plus or minus 0.9%) is remarkably similar. These values are almost identical to the relative rate of collagen synthesis in rabbit lung in the same age range. This technology should be applicable to answer specific questions regarding collagen synthesis and degradation in human lung disease.