Vaccines of non-pathogenic intestinal bacteria for oral administration have been used in the therapy of chronic and recurrent infections by the German Medical Association for Microbiological Therapy for over three decades. Three different oral bacterial vaccines were used in particular: 1. A sterile autolysate of non-pathogenic S. faecalis and E. coli; 2. a viable non-pathogenic S. faecalis vaccine, and; 3. a viable non-pathogenic E. coli vaccine. Clinical studies indicate the safety and efficacy of these bacterial products, and suggest the stimulation of immune activities and competitive capacities of S. faecalis and E. coli as mode of action. In animal experiments, orally administered intestinal bacterial vaccines enhance the resistance of mice against subsequent challenge with lethal doses of Salmonella typhimurium and Haemophilus influenzae. Mice were allowed access to a viable suspension of either S. faecalis or E. coli for at least 3 weeks. They were then challenged with either of the two unrelated pathogens. Both pre-treatment procedures conferred significant protection of the animals. The mechanism of this protective action appears to involve modification of white blood cell kinetics in the mice. The peritoneal resident cell population in mice is significantly increased by S. faecalis treatment.