Abstract: A method of determining the concentration of IgG antibodies in the biological fluids of mammals. The assay may be performed in a lateral flow cassette or dipstick format where dehydrated immobilized reagents are spaced along a membrane. A sample of a mammalian biological fluid is exposed first to an IgG complexing agent to yield a conjugate. The conjugate then moves along the membrane and is exposed to a standard mammalian IgG applied to the membrane at a test position. Binding of the IgG is indicated by a color change at the test position on the membrane. A control reagent of dehydrated anti-IgG complexing agent may be applied to the membrane at the control position spaced downstream from the test position. Interaction of the IgG complexing agent with the anti-IgG complexing agent forms a visible line at the control position on the membrane.