The adition of trisodium phophate 4.3% (Na3PO4. 12H2O 10 g. in 100 ml. of destiled watter) in equal parts to sputum for 24 hours, before the inoculation by the swab technique of Kudoh in modified Ogawa, substantially disminishes the contamination rate without altering the viability of M. tuberculosis, and facilitates the impregnation of the sputum in the swab.