In several previously reported studies a number of BCG strains, including those most widely used in vaccine production, were ranked according to their in vivo activity in various experimental models in rodents and to the local and allergic response that they provoked in children. In this report, 12 strains are ranked in terms of tuberculin conversion in guinea-pigs according to the minimum sensitizing dose. For 10 of these strains, this minimum dose is very low, ranging from 5 to 50 culturable particles. Thus the traditional practice of tuberculin-testing guinea-pigs that have been vaccinated with a full human dose of BCG (of the order of one million culturable particles) has such a low discriminating power that it is useless as a routine test for currently used vaccines. The ranking obtained in this model was largely but not completely in accordance with similar rankings in other models. The Tokyo strain and, to some extent, the London strain ranked comparatively lower than they do in children. The pronounced lack of virulence of the Prague strain was confirmed.