Microinjection techniques were used for the study of the translation of heterospecific messenger RNAs in living cells. Duck globin messenger RNAs, in the form of polysomes, messenger ribonucleoprotein particles, or free mRNAs, were injected into cells of human origin (HeLa), and the subsequent appearance and accumulation of avian globin were measured by specific immunofluorescence techniques. Globin accumulated within a high proportion of HeLa cells for at least 25 hr following injection, indicating the intracellular stability of the heterospecific globin messenger RNA and the fact that mRNA initially associated with more complex structures could be translated in the HeLa cytoplasm. In similar experiments, duck reticulocyte polysomes were shown to be translated in duck embryo cells but not in embryonic chick liver cells. The latter, however, could be shown to withstand the microinjection procedure as judged by their capacity to respond normally to inducers and inhibitors of the synthesis of alpha-aminolevulinic acid synthetase. HeLa cells injected with duck hemoglobin divide at normal or nearly normal rates following the injection, but their globin content diminishes, in contrast with the increasing globin content of cells receiving duck reticulocyte polysomes, mRNP particles, or free mRNA.