Several broad range antibiotics given to the dog in advance of producing hemorrhagic shock prolong the capacity to sustain a severe degree of hemorrhagic hypotension, and increase the survival rate following transfusion from 20% or less to 60% or more. Antibiotics given after shock is induced are much less effective, and fail altogether if given in advanced shock. The particular bacterial spp. responsible for irreversibility to transfusion in hemorrhagic shock in the dog were not identified, but flora invading from the intestine before and during shock appear to be involved. The protection afforded by antibiotics is due to the suppression of bacteria already present in the tissues or invading during shock.