Of 320 patients surveyed in a general hospital's medical wards during a 6-month period in 1981, 18 (5.6%) harboured enterobacteria which contained resistance plasmids conferring resistance to trimethoprim (Tp). At the beginning of the study period, Tp-containing therapy was not the first choice for treatment of infections caused by these bacteria and the incidence of plasmid-determined resistance was 10%. When Tp alone was used as the first choice of treatment of urinary tract infections and the use of this antimicrobial was correspondingly increased, the proportion of strains that contained Tp resistance plasmids (R-plasmids) decreased to 4%. Relatively more patients with Tp-resistant strains in the bowel had significant bacteriuria compared with those without Tp-resistant organisms. The continuing occurrence of Tp-R-plasmid containing strains in the gut was associated with continuing antimicrobial therapy; the strains generally disappeared after antimicrobial therapy was stopped.