1. Fowl osteoblasts grown in bulk tissue cultures in the presence of 14C-(L)-proline incorporated this amino acid into peptide linkage. A significant amount of the incorporated radioactivity was found in the hydroxyproline, glutamic acid, and aspartic acid fractions of the cultures.
2. The rate of formation of protein-bound 14C-hydroxyproline from 14C-(L)-proline was maximal in cultures grown for 15 hours and fell exponentially with the increasing age of the cultures.
3. 14C-(L)-glutamic acid was incorporated by the osteoblast cultures, but no significant amount was converted to hydroxyproline.