For decades, bacterial strains of low virulence were occasionally used in man to replace or to block colonization by the more virulent organisms and thereby prevent bacterial infection. This paper reviews the topic and presents recent information on the implantation of strain 215 α-hemolytic streptococcus (α-strep) in the nasopharynx of neonates in the intensive care unit. A single inoculation of strain 215 can change abnormal colonization of the pharynx to "normal" (α-strep predominant) in 48–72 h in most neonates. Following implantation, α-strep with strain 215 like characteristics fluctuate among naturally occurring strains of α-strep, sometimes persisting in dominance and sometimes decreasing rapidly as new strains appear. Strain 215 can survive in the pharynx during subsequent antibiotic therapy and can be recalled to dominance by such therapy. It seems remarkably stable in vivo. There is no evidence of its nosocomial spread in the nursery. Streptococcus with strain 215 like characteristics occurred naturally in 1–6% of neonates in our intensive care unit. No infection (disease) attributable to strain 215 occurred in implanted infants.