Summary Three methods for the biological assay of diphtherial toxoid have been presented. Due to economy of animals and other practical considerations, the method involving the skin test as the challenging dose has been selected as the method of choice. Prigge’s observations on the value of Lf determinations have been discussed. One provisional standard was used for testing all types of toxoid throughout this work. For convenience, an arbitrary unitage was established and the toxoids assayed are reported in terms of units.