less than that of the tubular container the function of the retaining means 13 is simply to retain the solid phase reagent 12 within the container 11 and thereby facilitate the separation of the adhered reaction components from those remaining in the assay medium at the conclusion of the assay reaction the retaining means 13 should be positioned proximal to the container opening 15 so as to permit maximum mobility of the solid phase reagent 12 within the container ideally the retaining means 13 should be fashioned so as to allow the removal of the assay medium while retaining the solid support in other words it would serve no purpose for the retaining means to be situated so that the retained solid phase reagent 12 would block the opening 15 of the container and prevent the removal of the assay medium the retaining means 13 could assume a variety of forms and configurations for instance a simple cap for the container 11 with a hole or a number of holes might be suitably employed a springlike device wedged within the container 11 near the opening would also function within the scope of the claimed invention the drawings illustrate two variations of one very practical approach crimping reducing the diameter of the container 11 by crimping at the or near the opening 15 is a convenient and inexpensive method.