Primary characterization test appHcable particularly to Gram-negative bacteria, colonies turning intensely violet on contact with 1 per cent tetramethyl-p-phenylenediamine dihydrochloride. Neisseria and Pseudomonas are strongly positive, but other genera share the characteristic. In Kovac’s version of the test a platinum loop is used to mix colony with reagent on filter paper; reaction should occur within 10 seconds.