Trent, Dennis W. (University of Oklahoma School of Medicine, Oklahoma City), and L. Vernon Scott. Colorado tick fever virus in cell culture. II. Physical and chemical properties. J. Bacteriol. 91:1282–1288. 1966.—Heat-inactivation kinetics for Colorado tick fever (CTF) virus grown in L cells indicated that more than one rate constant was involved for inactivation at each exposure temperature. An Arrhenius plot of the data indicated the inactivation rate constants to be dependent on the absolute temperature. The energy of activation, for thermal inactivation of the virus, was 17,289 calories per mole, with the Q₁₀ being 2.6. The optimal pH range for maintenance of CTF viral infectivity was determined to be 7.5 to 7.8. The infectivity of CTF virus was stable to freezing and thawing in diluents which contained: 50% calf serum, 20% glucose, 20% glycerol, 10% bovine serum albumin, 20 mm glutamine, and 2% gelatin. CTF virus replication was insensitive to inhibition by 5-fluoro-2′-deoxyuridine and 5-bromo-2′-deoxyuridine, whereas herpes simplex virus was markedly inhibited, as reported by others. Actinomycin D inhibited CTF virus replication when cells were pretreated for 24 and 12 hr prior to infection, but not when the inhibitor was added at the time of infection. The nucleic acid of CTF virus appears to be of the ribose type.