Biologically active bacteriophage T4 lysozyme is synthesized in vitro using RNA from T4-infected cells to program a protein-synthesizing system from uninfected cells (Salser et al., 1967). We have confirmed this finding, and have extended it to a number of T4 proteins characterized by their migration on acrylamide gel. Two widely used protein-synthesizing systems differ considerably in their suitability for such experiments. © 1969.