The biosynthesis of elastin was examined in matrix-free cells isolated by enzymic digestion of aortas from 17-day old chick embryos. After the cells were incubated with [14C]proline and then were rapidly boiled in buffer containing high concentrations of protease inhibitors and sodiumdodecyl sulfate, about one-quarter of the intracellular 14C-labeled protein was recovered as an elastin component with an apparent molecular weight of about 72 000. Examination of the medium from the cell suspension indicated that the largest elastin component secreted by the cells also had an apparent molecular weight of about 72 000. Pulse-chase experiments with intact aortas demonstrated that about two-thirds of the 72 000-dalton component disappeared in 2 h, apparently because it was converted to cross-linked fibers. When cross-linking was inhibited with penicillamine, the 72 000-dalton component persisted in the tissue 5 h. When cross-linking was inhibited with beta-aminopropionitrile, the elastin component of 72 000 daltons persisted for about 2 h, but thereafter it was gradually degraded to small peptides which were recovered in the incubation medium. The results suggest that elastin is secreted by cells in chick aorta as a polypeptide of about 72 000 daltons and that the secreted protein is incorporated into elastin fibers without cleavage to a protein of considerably smaller size.