A previous report from this laboratory showed that purified DNAs are transcribed after injection into the nucleus of Xenopus laevis oocytes (Mertz and Gurdon, 1977). Here we demonstrate that at least some of the RNA synthesized is translated within these injected cells to produce the correct protein products. Injection of Simian Virus 40 DNA into oocytes induces the synthesis of at least two proteins not normally synthesized in these cells. Using two-dimensional polyacrylamide gel electrophoresis and well characterized deletion mutants of SV40 that produce proteins of smaller size, we show that these two proteins are, indeed, the virus-coded proteins VP1 and VP3. Synthesis of VP1 and VP3 is inhibited by alpha-amanitin and, therefore, presumably mediated by a type II RNA polymerase. We also present evidence indicating that a histone-like protein is induced after the injection of a cloned plasmid DNA that codes for the Drosophila melanogaster histone proteins. This in vivo coupled transcription-translation system should be useful for identifying and mapping proteins coded by cloned eucaryotic DNAs.