The arena is square and shape and has no indestructible walls or barriers. It does, however, possess a destructible wall of green and orange enzymes on all four sides. The arena appears to take place on a desk or table. The floor is decorated with what appear to be drink stains, many slips of paper, and the logo for Xeno Industries. On the slips of paper are doodles and a to-do list for Doctor Nastidious. The table has a large purple square in the middle that is surrounded by the blue outer borders.
The Test Subject arena is the stage for level one in the campaign. It is unlocked for Multiplayer mode upon completing it in Campaign Mode. It is the easiest level as it only has one opponent, Blue, to defeat. It also has no hazards and only one stage element, that being the wall of enzymes that surrounds the stage. Enzymes will splatter into goo and disappear upon being bumped into. Bumping into the enzymes is essential for completing the stage as the player has to create a gap to push Blue off the edge.