Abstract: A method of binding an antibody with protein A cells is provided which includes a sequence of incubation and dilution steps to produce a preselected amount and concentration of antibody with a preselected distribution of antibody among the protein A cells. In addition, a method is provided for preparing an antibody entrapped porous matrix and apparatus which includes a specific porous matrix with a preselected position of antibody bound bacterium cells therein along with means for drawing fluids through the porous medium and means for facilitating the deposition of fluids onto the surface of the porous matrix. The apparatus and method is useful for testing for the level of progesterone in animal body fluids, such as milk, plasma, serum, whole blood and saliva.