P protein of phage lambda and dnaB protein of Escherichia coli were isolated from (a) bacteria containing an inducible lambda P gene on a plasmid, and (b) phage-lambda-infected bacteria. P protein from both sources copurifies with part of the dnaB protein during four purification steps. A highly purified preparation contains the multimeric dnaB and the P protein in a complex as revealed by glycerol gradient centrifugation. The complex is composed of two major polypeptides. Their molecular weights of 52 000 and 26 000 are identical to those previously determined for the dnaB and P polypeptides, respectively. The complex contains a DNA-dependent ribonucleoside triphosphatase activity which can be inactivated by anti-dnaB globulin. Both the dnaB complementing and the ribonucleoside triphosphatase activities are partially masked by the P protein as shown by their stimulation following a treatment with sodium chloride and N-ethylmaleimide.