Cytopathic effects in Vero cells following a neutralization test. Mixtures of sera dilutions and EV71 strain A104 virus were assayed on Vero cells. Cytopathic effects were examined after 7 days of incubation and neutralization titers were determined. Pre-challenge sera showed the NPfl-immunized samples (1:8 dilution) and NPt-VP11-100-immunized samples (1:8 dilution). 1:512 dilution was used for the post-challenge and the positive control sera. No serum was used in the negative control samples. Magnification = 200X.