Abstract The proteins from bacteriophage N4, dissociated with 1% SDS and 2% 2-mercaptoethanol, have been investigated by SDS polyacrylamide gel electrophoresis. Ten different polypeptide chains were identified in the purified virus. The molecular weights of the protein monomers estimated by coelectrophoresis with marker proteins ranged from 31,000 to 92,000 daltons. The moles of each structural polypeptide per mole of virion were also estimated by gel electrophoresis of N4 which had been uniformly labeled with 14 C-labeled amino acids. The ten structural polypeptides of N4 account for about 25% of the genetic information contained in the virus DNA. Pulse-chase experiments performed on infected cells in order to determine whether certain N4 structural components are synthesized as larger precursors gave negative results.