KB cell monolayers, infected with dengue-2 virus and maintained under liquid medium, give rise to persistently virus-yielding cultures regardless of the input dose of virus or of the intensity of initial cytopathic changes. Persistently infected cultures continuously yield 0.3–0.4 LD₅₀ per cell for periods of at least 3 years.

Conditions are defined under which maximal cytopathic effects (CPE) of dengue-2 virus are obtained in KB cell cultures. Based on the finding that agar prevents CPE and demonstrable viral multiplication, a plaque assay procedure for dengue-2 virus has been developed which involves the use of methylcellulose overlay medium. As used, this method gives highly reproducible results with an average efficiency equivalent to that of the LD₅₀ titration in mice. Preliminary tests with dengue-1, yellow fever, Japanese B, Saint Louis, and Murray Valley encephalitis viruses suggest the general usefulness of the methylcellulose overlay technique for group B arborviruses.