Post-infection chloroquine treatment reduces SARS-CoV infection and spread. Vero E6 cells were seeded and infected as described for Fig. 1 except that chloroquine was added only after virus adsorption. Cells were maintained in Opti-MEM (Invitrogen) containing chloroquine for 16–18 h, after which they were processed for immunofluorescence. (A) The concentration of chloroquine is indicated on the top. (B) Percent inhibition and SEM were calculated as in Fig. 1B. (C) The effective dose (ED50) was calculated using commercially available software (Grafit, version 4, Erithacus Software).