The enhancement of vesicular stomatitis virus plaques on human embryonic lung cells in the presence of Tween 80 or Aquasol A was studied to determine the optimal conditions for the enhancement. Enhanced numbers and sizes of vesicular stomatitis virus plaques occurred when Aquasol A or Tween 80 was added to the cell culture 30 min before virus adsorption but not when added after adsorption These substances did not have a direct effect on the virus and did not have an effect on virus adsorption or penetration. A few other viruses and cell systems were also studied to determine if enhancement would extend to other viruses and cell systems. The cell system seemed to be important since enhancement of vesicular stomatitis virus plating efficiency did not occur on chicken embryo cells. However, the virus was also important since vaccinia virus plating efficiency was not enhanced on the human embryonic lung cell. The greatest enhancement encountered was the increase in the plating efficiency of Friend leukemia virus on S+L- cells.