Abstract 1. 1. Protein synthesis in a cell-free system from Musca domestica L. has been shown to occur by the pathway involving amino acyl-adenylates and amino acyl-soluble ribonucleic acid as intermediates. 2. 2. The system requires an ATP-generating system, a divalent cation and a monovalent cation, guanosine-5′-triphosphate and microsomes or ribosomes and pH 5 enzymes for optimal activity. 3. 3. The requirement of guanosine-5′-triphosphate is not evidence in larval specimens. 4. 4. An insecticide-susceptible strain did not show a dependence on the pH 5 fraction, suggesting that the corresponding enzymes may be tightly bound to ribosomes. 5. 5. Phenylthiourea is required for optimal activity in pupal preparations, but its effect is not significant in other developmental stages.