Transient streptococcal bacteriæmias are a frequent sequel to dental extractions especially when the mouth is the seat of severe chronic gum infection. Bacteria may also gain admission to the blood-stream in such cases irrespective of operative procedures and probably as the result, in many instances, of minor degrees of gum injury such as is produced by biting on a loose tooth. Acute apical infections do not appear to be especially associated with blood infection of this kind, the focus of infection here apparently being effectively "walled off" by the associated inflammatory reaction.Of the two factors, infection and trauma, involved in the production of these post-operative bacteriæmias, infection appears to be the more important since, when it is marked, very slight degrees of gum injury are sufficient to produce blood-stream invasion. In the complete absence, however, of the type of trauma induced by the "rocking" of a tooth during its removal, extraction may be accomplished without producing a heavy bacterial shower in the blood.Usually these transient bacteriæmias produce no permanent ill-effect, but there is some evidence that, occurring in subjects with abnormal heart valves, they may lead to subacute infective endocarditis. Thirteen cases are reported where the valvular infection appeared to result from a post-operative dental bacteriæmia.Prevention of such bacteriæmias may be achieved by the reduction or elimination of infection and trauma. Complete elimination of the gum infection is difficult although preliminary treatment of the gum margin by some measure such as cauterization may lessen it and lead to a reduction of the post-operative bacterial shower. Similarly, by manipulating an infected tooth as little as possible during its extraction the incidence or degree of blood infection may be decreased.