A device for assaying biological fluids for molecules contained therein comprising a container, material situated in the container for absorbing fluid and in communication with an antibody or antigen impregnated matrix, wherein the matrix is accessible to the exterior of the container through a funnel shaped aperture in the roof of the container. Further features include a chemical drying agent associated with the container for absorbing moisture, thereby preventing inactivation of the assay reagents, and a filter situated above the antibody or antigen impregnated matrix, and in communication with the matrix through the aperture in the roof of the container. The filter removes interfering substances present in the biological fluids, and provides protein blocking agents to the matrix material for decreasing the background of the assay.