In vitro systems for the aminoacylation of transfer ribonucleic acid (tRNA) and for polypeptide synthesis have been constructed from young (12-h cultures, not producing actinomycin) and old (48-h cultures, producing actinomycin) cells of Streptomyces antibioticus. When Escherichia coli aminoacyl-tRNA synthetases were used to acylate S. antibioticus tRNA's, it was observed that, per absorbance unit of tRNA, the tRNA's from 48-h cells had a lower ability to accept the amino acids, leucine, serine, pheynlalanine, methionine, and valine than did the tRNA's from 12-h cells. Individual differences were observed between aminoacyl-tRNA synthetases from 12-h cells and those from 48-h cells with respect to the rate and extent of aminoacylation of E. coli tRNA with the five amino acids listed above. In vitro systems for the synthesis of polyphenylalanine have been constructed from 12- and 48-h cells. Ribsomes and soluble enzymes from 12-h cells are more efficient than those from 48-h cells in supporting polyphenylalanine synthesis, and, although the activity of both systems can be stimulated by the addition of E. coli tRNA, the higher level of incorporation observed in the unstimulated 12-h system (ribosomes and soluble enzymes) is maintained. Indeed, the difference in capacity for polyphenylalanine synthesis between in vitro systems from 12- and 48-h cells is greater when the systems are maximally stimulated by E. coli tRNA. Cross-mixing experiments reveal that enzymes from 48-h cells support a slightly higher level of polyphenylalanine synthesis than enzymes from 12-h cells with ribosomes from either cell type, and that the ribosomes are the primary agents responsible for the decreased efficiency of the in vito system from 48-h cells are compared with that from 12-h cells. To determine whether ribosome-associated factors were responsible for the relative inefficiency of the ribosomes from 48-h cells in translation, salt-washed ribosomes from 12- and 48-h cells were examined for their abilities to catalyze polyphenylalanine synthesis. Even after salt washing, ribosomes from 12-h cells were about five times higher in specific activity (counts per minute of polyphenylalanine synthesized per absorbance at 260 nm of ribosomes) than equivalent amounts of ribosomes from 48-h cells. Analysis of the proteins of salt-washed ribosomes of the two cell types by acrylamide gel electrophoresis suggests that the relative amounts of individual proteins present on ribosomes from 12-h cells are different from the amounts present on ribosomes from 48-h cells. These results are discussed in terms of the regulation of translation in S. antibioticus.