SUMMARY The survival of ‘marked’ strains of Escherichia coli bearing antibiotic-resistance-transfer (R) factors was studied in the faeces of healthy human volunteers for 7 days after ingestion. Two R factors, determining resistance to ampicillin/chloramphenicol/kanamycin/streptomycin/sulphonamides and to chloramphenicol/streptomycin/sulphonamides/tetracyclines were used in 14 experiments on 12 persons. Strains without R factors served as controls. Both R-factor-bearing and R-factor-free strains were identified by chromosomally determined resistance markers. The R-factor-bearing organisms were isolated from the faeces in fewer numbers than the controls in 52 out of the 54 samples studied. In the two samples in which the reverse phenomenon was observed the effect was not marked. The fall in the number of R-factor-bearing organisms was generally progressive, and 4 days after ingestion the mean faecal population of R-factor-bearing organisms was only 0·5% of the control organisms. The relative decline in the R-factor population probably resulted from impaired vitality of organisms bearing these plasmids rather than from loss of R factors from the cell.