AFTER capsulated B. anthracis had grown in a modified synthetic medium of Brewer et al.1, a mixture of growth products was isolated from the glass-filtered medium which gave a precipitate with copper sulphate. Crude polyglutamyl peptide was obtained from this precipitate2. An attempt was made to separate the components of the original mixture of growth products by paper electrophoresis using a tank as described by Flynn and Mayo3, and it was found that the peptide could not be detected using naphthalene black, bromo-phenol blue, ninhydrin or ultra-violet light4. We found it could be detected by taking advantage of the acidic nature of the peptide, using the following technique : (1) paper strips with the peptide applied are run in a suitable buffer, for example, veronal/veronal sodium at pH. 8.6 or acetic acid/sodium acetate at pH 4.5, and dried at 100°; (2) the buffer is washed out with two changes of 80 per cent ethanol; (3) the strips are treated with N/50 hydrochloric acid in 90 per cent ethanol; (4) the acid is thoroughly removed with ethanol.; (5) after drying, the strips are dipped into N/300 alcoholic sodium hydroxide containing 0.04 per cent bromo-cresol purple, which is repeated if necessary until colour differentiation is obtained, and the strips are blotted.