An infant with severe combined immune deficiency was delivered by Caesarean section and continuously maintained in an isolator in an effort to restrict microbial contact. However, during the past 5 years the child came in contact with at least 54 different microbial contaminants. His skin autoflora was similar to the reference group of healthy male adults in numbers of different species and the number of viable cells present per square centimeter of surface area. The subject's autoflora differed from the reference group in that significantly fewer anaerobic species were recovered from the patient's mouth and feces. The populations on the skin and in the upper respiratory tract were almost exclusively composed of staphylococci and micrococci, although several species that commonly inhabit these areas were infrequently isolated and failed to colonize. The fecal autoflora was dominated by staphylococci, and bacilli clostridia recovered in typical numbers. Enterobacter agglomerans, not commonly recovered from the reference group, was a frequent inhabitant of the mouth and feces of the child. The fact that this severe combined immune-deficient patient has remained disease free in the presence of this microbial population illustrates the noninvasive nature of the bacteria present, as well as the importance of unaffected components of the child's immune defense mechanism.