An in vivo model is the only means whereby organisms of low virulence can be investigated. A 20% surface area, third degree burned rat has been utilized in the study of burn wound sepsis. Studies with it show that the mortality is directly proportional to the bacterial concentration in the burn wound. Following topical seeding with Pseudomonas aeruginosa, 72 hours pass before lethal levels are attained in the subeschar tissue. Various topical and systemic agents are evaluated on their ability to control the number of bacteria, to reduce mortality, and to affect insensible water loss. Studies determining the lung's ability to clear bacteria show that it is related to the total number of bacteria in the wound. This suggests a reason for the failure of reduction in mortality in burns of over 60% of body surface.