A polypeptide - synthesising cell-free system directed by Poly-U or Turnip Yellow Mosaic Virus-RNA was derived from imbibed seeds of Phaseolus aureus; this in vitro system was dependent upon exogenous tRNA. The Poly-U-directed system functioned in the presence of tRNAs from P.aureus, Vicia faba and yeast, whereas TYMV-RNA was translated only in the presence of tRNAs from P.aureus or V.faba. This translation barrier was related to the inability of the P.aureus high-speed supernatant enzyme fraction to charge various "protein" amino acids to yeast tRNA under uniform conditions. Such incompatibility did not exist in the Poly-U system where incubation conditions were constructed to favour only one amino acid, i.e. phenylalanine. The P.aureus enzyme fraction promoted esterification of valine to TYMV-RNA. More rigorous conditions were required for the translation of Poly-U in a Transfer System with yeast tRNA than with V.faba tRNA. Poly-U and TYMV-RNA competed for ribosomal binding sites. The characteristics of synthetic and natural templates in amino acid incorporation were correlated.