Dimethyl sulfoxide (DMSO) incorporated into an agar overlay containing DEAE-dextran enhanced plaque formation in Vero cells by Naples sandfly fever virus passaged in mouse brain or Vero cell cultures. No plaques were visible when DMSO was used without the DEAE-dextran, some plaques were barely visible (<0.5 mm) when DEAE-dextran was used without the DMSO, and up to 10-fold more plaques were clearly visible (0.5–1.5 mm) when both chemicals were used. The combined enhancing effect of DMSO and DEAE-dextran was also observed with mouse brain passaged, but not Vero passaged Sicilian sandfly fever virus. Other Phlebotomus group viruses produced big plaques (3–5 mm) and did not require DMSO for plaque formation, although an increase in plaque clarity was obtained with DMSO for some of them. Plaque reduction neutralization tests were assayed successfully under agar containing DMSO. The alphavirus Sindbis produced slightly larger plaques under agar containing DMSO, but there was no effect on clarity or size of plaques produced by the flavivirus dengue-2.