Abstract: A method for detecting the presence of Mycobacteria in a fluid or tissue which comprises mixing the fluid or tissue containing a secretory product of Mycobacteria with a complex of a tracer-containing molecule and a binding macromolecule having reversible binding affinity for the tracer-containing molecule and detecting the tracer-containing molecule, wherein the tracer-containing molecule is a charcoal-adsorbable protein from Mycobacterium tuberculosis which has a molecular weight of 20,000-30,000 and which is immunochemically stable from 4.degree. C. to 250.degree. C. and has a pH range from 3.0 to 9.0. The method is particularly applicable to the detection of infectious tuberculosis in humans and determining the antibiotic sensitivity of infecting Mycobacteria.