A cell-based assay for assessing the functional activity of a canine or feline IL-31 inhibitor candidate is provided. Such a canine or feline IL-31 inhibitor candidate can be an isolated antibody that specifically binds to at least one of canine Interleukin-31 (IL-31) or feline IL-31. Such antibodies can be in the form of diagnostic and/or veterinary compositions useful for treating a pruritic and/or allergic condition in dogs or cats.