The inventions relates to compositions and method for determining the absolute counts of cells per unit volume of a sample. Such a method comprises: (a) providing a container containing (i) a predetermined quantity of microparticles; and (ii) a cell-binding agent; in which the microparticles are disposed in or on a matrix which adheres to at least one wall of the container such that substantially all the microparticles are thereby attached to the container; (b) adding a known volume of sample to the container; (c) determining the ratio of microparticles to cells by counting microparticles and cells in a volume of the sample; and (d) determining the absolute count of cells by multiplying the number of cells per microparticle by the concentration of microparticles in the sample. Preferably, the matrix retains substantially all the microparticles in or on the container during routine handling, including inversion, of the container, in the absence of mechanical retaining means such as a retainer grid in the container.