A method and kit for assaying a cell sample for the presence of at least a threshold number of cells of a given type are disclosed. The kit includes an assay device having a sample chamber for receiving the cell sample and an elongate collection chamber containing a selected-density and/or viscosity medium and having along its length, a plurality of cell-collection regions, and particles which are capable of specific attachment to cells of the selected cell type, and which are effective, when attached to the cells, to increase the density or magnetic susceptibility of the cells. In operation, particle-bound cells and particles in the cell sample are drawn through the elongate collection chamber under the influence of a gravitational or selected centrifugal or magnetic-field force until the particle-bound cells and particles completely fill successive cell-collection regions in the collection chamber. Indicia associated with at least one collection regions indicates a concentration of cells of the selected type effective to at least partially fill that collection region.