||Radioactive labeling tyrosine peptides of reticulocytes- cell free extracts. When a radioactive amino acid is added to a cell or cell-free system synthesizing globin, radioactivity appears first in the nascent chain, then, only after a lag, in released protein. The radioactive label will not appear in an NH2-terminal amino acid in completed, released globin chains until a ribosome has traversed the mRNA from the NH2- to COOH-terminal end. The translation time for a mRNA, strictly, the elongation plus termination time, is taken as the time interval required for the specific activity of the NH2-terminal residue in released protein to reach the specific activity that the same residue had in the total (nascent plus released) protein at the beginning of the time interval (Fig. 2). If, instead, one measures the COOH-terminal residue one obtains the time required for release of the completed globin chain from the ribosome.