1. If normal guinea pigs are confined with an equal number of tuberculous cage mates the incidence of "contact" tuberculosis is increased by crowding. This is probably due largely to an increase in the amount of tubercle bacilli available in the more crowded cages, although no constant relationship could be established between the intensity of the exposure and the incidence of tuberculosis acquired by contagion. Other factors must be determined.
2. If guinea pigs are inoculated intraperitoneally with a given quantity of human tubercle bacilli and distributed in different degrees of crowding, the duration of survival is shortened in the more crowded animals, and the incidence of chronic types of tuberculosis is greater among the less crowded animals.