The aerobic gram-negative intestinal flora of two individuals living in close proximity was followed for 17 months. One of these persons was receiving a prolonged tetracycline treatment for acne vulgaris and was colonized by tetracycline-resistant Escherichia coli strains throughout the survey. The other person studied received no antibiotics during the period in question, but was frequently found to excrete a number of E. coli strains harboring tetracycline-resistant plasmids. The two E. coli strains (O75.H7 and O23.H16) excreted from both persons most frequently were indistinguishable, and so were the R-plasmids they carried. This suggests that R-plasmid-carrying E. coli may spread from individuals under treatment to close relatives that have not been treated.