A method to prevent endogenous infections in patients and laboratory animals with an impaired immune capacity is the total decontamination of the gastrointestinal tract. This is performed by oral administration of nonabsorbable antibiotics. One of the disadvantages of total decontamination is the fact that especially after termination of this treatment the decontaminated individual is easily colonized with organisms from the environment. This is the result of the elimination of the anaerobic part of the microflora, which is responsible for the so-called colonization resistance (CR). This CR is the resistance against exogenous microorganisms to colonize the gastrointestinal tract. In the absence of an anaerobic flora, the colonizing microorganisms can reach abnormally high faecal concentrations, thus increasing the risk for infection. In mice, the implantation of an anaerobic, mouse-derived flora after termination of total decontamination resulted in the restoration of a good CR, as could be shown by orally challenging the animals with a strain of Escherichia coli. Therefore, an anaerobic microflora, free of potentially pathogenic microorganisms, which was isolated from a healthy human volunteer was administered to monkeys and patients after a decontamination period in an attempt to restore CR. In the monkeys, this human donor flora (HDF) did not reduce the faecal concentration of microorganisms that had colonized the gastrointestinal tract before the donor microflora had been established, in contrast to the findings in some of the patients. Qualitative analysis of the microflora of patients which were contaminated with the human donor flora showed that the CR is not of the same quality as is found in healthy individuals. This can be the result of the impaired immune capacity of the patients at the time of HDF implantation. However, the results obtained show that implantation of the HDF in monkeys and patients after a decontamination period allows reconventionalization without an undue risk of microbial infection.