Diarrhoea was reproduced by inoculating 5-week-old New Zealand White rabbits orally with 2 X 10(6) CFU of E. coli 0.15:H-. The strain produced diarrhoea in all the dosed rabbits 6.25 +/- 2.71 (mean +/- S.D.) days after infection (p.i.). Mortality was high. Sequential examination of the intestines by light microscopy showed the strain to attach first to the tips of the lymphoid follicle epithelium of Peyer's patches 24 h p.i., but not to other sites until 3 days p.i. From 3 to 14 days p.i. the strain caused an acute intestinal inflammatory response and large numbers of E. coli adhered to the enterocytes of ileum, caecum and colon. Colonized cells became rounded and were desquamated. Severe oedema of the caecal lamina propria was often present. Colonization of the intestine, as shown by rectal swab cultures, reached a maximum 4 to 7 days p.i. and did not decrease until 14 days p.i.