Then, while repetitions of Koch's experiment, upon which his announcement was based, of inoculating calves and young cattle with _human_ bacilli have proved that a certain number of them can be, under appropriate circumstances, made to develop tuberculosis, that number has never been a large percentage of the animals tested, and in many cases the infection has been a local one, or of a mild type, which has resulted in recovery. Lastly, while a number of bacilli, with _bovine_ culture and other characteristics, have been recovered from the bodies of children dying of tuberculosis, and these bacilli have proved virulent to calves when injected into them, yet, as a matter of historical fact, the actual number of instances in which children or other human beings have been definitely proved to have contracted the disease from the milk of a tuberculous cow is still exceedingly and encouragingly small. A careful study of the entire literature of the past twenty years, some three years ago, revealed _only thirty-seven cases_; and of these thirty-seven Koch's careful investigations have since disproved the validity of nine. On the other hand, it is anything but safe to accept Koch's practical dictum and neglect the meat and milk of cattle as a source of danger in tuberculosis. First, because the degree of our immunity against the bovine bacilli is still far from settled; and, second, because, while bacteriologists are fairly agreed that the _avian_, the _bovine_, and the _human_ represent three distinct and different variations, if not species, of the bacillus, they are almost equally agreed that they are probably the descendants of one common species, which may possibly be a bacillus commonly found upon meadow grasses, particularly the well-known timothy, and hence very frequently in the excreta of cattle, and known as the _grass bacillus_ or _dung bacillus_ of M[oe]ller. This bacillus has all the staining, morphological, and even growth characteristics of the tubercle bacillus except that it produces only local irritation and little nodular masses, if injected into animals. Our knowledge of its existence is, however, of great practical importance, inasmuch as it warned us that in our earlier studies of the bacilli contained in milk and butter we have been mistaking this organism for a genuine tubercle bacillus. As a consequence, of late years our tests for the presence of tubercle bacilli in milk are made not only by searching for the organism with the microscope, but also by injecting the centrifugated sediment of the infected milk into guinea pigs, to see if it proves infectious. Many of our earlier statements as to the presence of tubercle bacilli in milk and butter are now invalidated on this account.