Pyrazinamidase activity was tested in 1,033 strains of 15 different genera of Enterobacteriaceae. The test was performed by a simple method which reveals the production of pyrazionic acid by a heavy bacterial suspension in 0.2 ml of a 0.5% pyrazinecarboxamide solution in Tris-maleate (0.2 M, pH 6) buffer after incubation for 18 h at 37 degrees C. Positivity was revealed by the development of a pink to red color after addition of 2 drops of 1% extemporaneously prepared ferrous ammonium sulphate aqueous solution. Pyrazinamidase activity proved to be a useful characteristic for the differentiation of some species from other enterobacteria devoid of this property. Because of its simplicity, this test is suitable for its introduction into the miniaturized diagnostic kits for the identification of enterobacteria.