Diphtheria antitoxin assay by flocculation. The antitoxin under test is serially diluted and mixed in tubes each containing ‘1 Lf’ of toxin, the dilution which first flocculates contains 1 Unit of antitoxin. (The 1 Lf of toxin is first determined by a reversal of the process testing with standard antitoxin of 1 Unit strength.)