The occurrence in this country of an epizootic disease among field-voles (Microtus agrestis), resembling tuberculosis in its anatomical features but due to an acid-fast bacillus different from the tubercle bacilli found in other species of warm-blooded animals, has raised several interesting questions. One of these is the relative susceptibility of the field-vole, and other species of wild rodents belonging to the family Muridae, to the bovine, human and avian types of tubercle bacilli and the possibility of using any of these species in place of the rabbit for differentiating between the three types. Some work on the subject was done by me in the year 1923, the rodents tested being field-voles and wood-mice (Mus sylvaticus). These experiments, reported in 1937, showed that the field-vole is highly susceptible to bovine bacilli, and can also be infected with human bacilli, though with less certainty and less severity than with bovine bacilli. The wood-mouse is susceptible to bovine and human bacilli, perhaps more so to the former than to the latter, and stands in an intermediate position between the vole and the white mouse in its susceptibility to mammalian tubercle bacilli.