A simple method is described for converting a standard rabbit reticulocyte cell-free extract (lysate) into an mRNA-dependent protein synthesis system. The lysate is preincubated with CaCl2 and micrococcal nuclease, and then excess ethyleneglycol-bis(2-aminoethylether)-N,N'-tetraacetic acid is added to chelate the Ca2+ and inactivate the nuclease. Lysates treated in this way have neglibible endogenous amino acid incorporation activity, but 75% of the activity of the original lysate can be recovered by the addition of globin mRNA. The efficiency of utilisation of added mRNA and the sensitivity of the system are both very high. No residual nuclease activity could be detected, and the tRNA is functionally unimpaired. Several different species of mRNA have been shown to be translated efficiently into full-sized products of the expected molecular weight up to about 200000, and there is no detectable accumulation of incomplete protein products. The efficient translation of RNA from two plant viruses (tobacco mosaic virus and cowpea mosaic virus) required heterologous tRNA.