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Localization of the Enzyme and its Template To investigate whether the DNA polymerase and its template were associated with the virions, a R-MLV sus- pension was centrifuged to equilibrium in a 15-50 per cent sucrose gradient and fractions of the gradient were assayed for DNA polymerase activity. Most of the activity was 25 20 2 15 3H-TMP incorporated (c.p.m. x 10-3) 10 3 5 4 30 60 90 120 Min Fig. 1. Incorporation of radioactivity from *H-TTP by the R-MLV DNA polymerase in the presence and absence of ribonuclease. A 1.5-fold standard reaction mixture was prepared with 30 ag of viral protein and "H-TTP (specific activity 950 c.p.i. per pmole). At various times, 20 #d. aliquots were added to 0-5 ml. of non-radioactive 0.1 M sodium pyrophosphate and acid insoluble radioactivity was determined. For the preincubated samples, 0.06 ml. of H,Q and 0.01 ml. of R-MLV (30 ug of protein) were incubated with or without 10 ag of pancreatic ribonuclease at 22" C for 20 min, chilled and brought to 0-15 ml. with a concentrated mixture of the components of the assay system. Curve ], no treatment; curve 2, preincubated; curve 3, 10 pg ribonuclease added to the reaction mixture; curve 4, preincubated with 10 ag ribonuclease