Vero E6 cells are commonly used for in vitro studies of the severe acute respiratory syndrome-associated coronavirus (SARS-CoV) and for antiviral evaluation purposes. A better understanding of the SARS-CoV growth kinetics in Vero E6 cells is crucial to help elucidate the mechanism of antiviral activity of selective antiviral agents. In this study, the growth kinetics of SARS-CoV in Vero E6 cells were studied by quantitation of intra- and extracellular viral RNA load as well as extracellular virus yield at different time points post-infection. At 12 It post-infection, the intracellular viral RNA load was 3 x 10(2)-fold higher than at the time of infection, and the extracellular viral RNA load was increased with a factor of 2 x 10(3). Intracellular viral RNA levels started to rise at 6 It post-infection. One hour later (at 7 It post-infection), the levels of extracellular SARS-CoV RNA also began to rise. This was corroborated by the fact that infectious progeny SARS-CoV also first appeared in the supernatant between 6 and 7 It post-infection. At 12 h post-infection, SARS-CoV reached titers in the supernatant of 5.2 x 10(3) CCID50/ml. (c) 2005 Elsevier Inc. All rights reserved.