A C6/36 cell culture persistently infected by dengue 2 virus was established and retained for over 20 weeks. No CPE was observed at any stage of infection. However, some giant cells up to 2- or 3-fold the diameter of normal cells appeared in the established culture. Extracellular virus titers fluctuated with a periodic tendency while the antigen level remained constant, suggesting some viral particles evolved into noninfectious defective interfering particles. Plaque size tended to change during the passage of cultures. The passage 8 virus mainly produced plaques relatively smaller than those produced by the passage 1 virus. Nevertheless, the plaque sizes produced by the passages 18 virus were somewhat mosaic. A temperature-sensitive mutant was detected in the virus released from passages 8 and 18. In addition, passages 8 and 18 produced the lowest virus titers. When persistently infected viruses were grown in Vero cells at 37 degrees, at passage 1 there was a high titer which decreased 5 days postinoculation, whereas at passages 8 and 18 titers were undetectable until 4 days postinoculation. It seems that dengue 2 virus persistently infected in C6/36 cells could have been genetically altered. This will be tested by nucleic acid analysis of the viruses.