A limited review of the results of using comparative reciprocal intradermal mycobacterial sensitin (eRIS) testing with guinea pigs for the classification and identification of mycobacteria is presented. The technical procedures and the materials used in CRIS testing are referred to only briefly. Strains of mycobacteria that grow well on a synthetic nonimmunogenic medium can be identified or classified at the species level with this method. At present some 50 species of mycobacteria can be identified by CRIS testing. With this method alone, delineation between the genera Mycobacterium and Nocardia (or between subgenera of Mycobacterium proposed earlier) and differentiation between individual strains of Mycobacterium tuberculosis, Mycobacterium bovis, and Mycobacterium africanum are not possible. In a blind study that utilized CRIS testing, a total of 10 strains of Mycobacterium avium complex serovars 1, 2, 4, and 6 and one strain of serovar 9 were identified as M. avium; a total of five strains of serovars 14, 16, 17, and 20 and one strain of serovar 12 were identified as Mycobacterium intracellulare; a total of seven strains of serovars 41,42, and 43 and two other strains of serovar 9 were identified as Mycobacterium scrofulaceum; and two other strains of serovar 9 appeared to be distinct from M. avium and from the other two species just mentioned (author's unpublished data). CRIS tests cannot be used at present for the identification of mycobacteria that cannot be cultivated.