The percentage of uncoating of monkeypox virus (MPV) in CV-1 cells measured by sensitivity of viral DNA to deoxyribonuclease was 24, 46, 53, 48, and 54, at respective 2, 3, 4, 5, and 6 hours post infection. Cores began to appear 1/2 to 1 hour post infection and subsided 3–4 hours post infection to yield naked DNA. The uncoating process was not inhibited by cytosine arabinoside. In cells pretreated with actinomycin-D and infected with MPV in the presence of 2 μg/ml actinomycin-D virion and core peaks were observed but no naked DNA appeared. The time required for a single step growth curve was 12 hours. At 2 hours post infection RNA slightly smaller than 16 S was synthesized. At 4 hours post infection 10 S to 20 S RNA was synthesized; 22 S at 6 to 10 hours. Nuclear RNA synthesis was suppressed.