Phage SP RNA-dependent RNA polymerase (SP replicase) was purified from Escherichia coli infected with RNA phage SP. The enzyme was found to be composed of four non-identical polypeptides, i.e. subunits I, II, III, and IV with molecular weights of 74,000, 69,000, 47,000, and 36,000 daltons, respectively. As in the case of phage Qβ replicase, the largest polypeptide is identical with the ribosomal protein S1, and subunits III and IV with polypeptide chain elongation factors EF-Tu and EF-Ts, respectively. This is based on the behaviour of the subunits on SDS-polyacrylamide gel electrophoresis, isoelectric focusing and immunological cross-reaction. Subunits I, III, and IV of SP replicase are derived from the host cell, while subunit II is coded by phage RNA genome. The striking coincidence of the composition and entity of the structural components of SP replicase with those of Qβ replicase may indicate the structural and functional requirements of host-derived polypeptides in RNA replicase. The binding activity of SI (in 70S ribosome complex) to poly (U) is retained in SP replicase complex. In contrast, the GDP binding activity of EF-Tu is masked in SP replicase. It is concluded that SI is required functionally whereas EF-Tu-EF-Ts are required structurally in RNA replicase.