A cell-free translation system, from the yeast Saccharomyces cerevisiae, has been used to develop an in vitro assay for yeast UGA, ochre and amber suppressors. Amber suppression was assayed by read-through of the brome mosaic virus coat protein cistron UAG terminator. UGA suppression was assayed by read-through of the rabbit beta-globin UGA terminator and ochre suppression by read-through of the rabbit alpha-globin mRNA UAA terminator. Ochre suppression was increased 3-fold when the globin mRNA was heat denatured prior to translation; this was due to an increase in the synthesis of alpha-globin relative to beta-globin. Amber suppression was more efficient in vitro (46%) than ochre suppression (14%). UGA suppression was also highly efficient in vitro, reaching almost 100% using a purified UGA suppressor tRNA from Schizosaccharomyces pombe. Unfractionated yeast tRNA, from a sup+ strain, contained a tRNA species able to suppress UGA termination codons in vitro, but no tRNA species able to suppress either UAA or UAG was found. This homologous in vitro assay for yeast nonsense suppressors will allow, for the first time, an approach to the biochemical analysis of yeast mutants that modify the efficiency of nonsense suppression in vivo.