Abstract: This invention relates to an improved grid for use in counting the bacterial colonies present in defined areas of a spirally-deposited liquid sample containing steadily decreasing concentrations of a chemical to be tested, such grid being characterized by a transparent overlay upon which is imprinted a spiral defined by outlining the side margins of its convolutions while leaving the area therebetween blank, the spiral including at least two complete revolutions with common margins therebetween positioned to overlie the deposition spiral, the convolutions of the grid spiral being of constant width and not less than approximately three combined deposition spiral convolutions, the rate of divergence of the grid spiral convolutions being greater than those of the deposition spiral such that the former intersect the latter at least twice each revolution, and with at least a portion of the grid spiral being segmented to divide same into a plurality of discrete areas positioned to overlay areas of the deposition pa