Abstract: The invention concerns a biological assay device using a reaction between an analyte present in a fluid and a reagent capable of forming a complex with said analyte, wherein the analyte is in the form of cellular elements, said device comprising at least a reaction container (1) provided with a reaction zone (2) wherein are introduced the fluid and the reagent, and with an immobilizing zone (3) whereon is fixed a substance (4) capable of specifically binding with the possibly formed complexes, the reaction zone (2) and the immobilizing zone (3) being separated from each other by a material layer (8), said material being capable of shifting from one first state wherein the layer (8) is substantially impermeable to a second state wherein the layer (8) is capable of allowing through the cellular elements, whether complexed or not.