Test of the ability of growing mycobacteria to split ester sulphates—a property of many mycobacteria but not of Myco. tuberculosis. Dipotassium phenolphthalein sulphate in Dubos Tween medium is inoculated and incubated for a time adequate for good growth of the strain being tested. On alkalizing the medium, free phenolphthalein (positive result) is shown by red colour.