The enteric flora of a group of antibiotic-treated patients and two groups of non-antibiotic-treated individuals were studied for antibiotic resistance patterns and presence of transferable R-factors. The former group consisted of 100 hospitalised patients, the latter two groups consisted of 200 hospitalised patients and 200 individuals from rural communities in Jamaica. The occurrence of multiple-resistant organisms with transferable R-factors was significantly higher in the antibiotic-treated group. R-factors were also more common in the hospitalised, non-treated group than in the non-hospitalised, non-treated group. Follow-up studies on hospitalised patients showed a tendency to increased occurrence of R-factors with duration of hospitalisation, whether the patients were treated with antibiotics or not. These studies have shown that both antibiotic treatment and hospitalisation promote the occurrence of R-factors.