Strains of Campylobacter jejuni, isolated from human gastro-intestinal infection and inoculated orally into 1-day-old chicks, colonised the alimentary tract (caecum) well. There was evidence of invasion from the intestine to the spleen. Oral inoculation with some but not all strains of C. jejuni 24 h earlier (within 12 h of hatching) prevented establishment by challenge strains administered orally 1 day later. One strain which was less able to colonise the gut was less inhibitory than other strains. Precolonisation of newly hatched chicks with a strain of Salmonella typhimurium had no inhibitory effect on establishment by the challenge strain of C. jejuni and may even have exacerbated it. Inhibition of multiplication of a nalidixic acid-resistant mutant of a C. jejuni strain was prevented when it was added to a stationary-phase broth culture of the antibiotic-sensitive parent strain and the mixed culture re-incubated. (C) 2000 Published by Elsevier Science B.V. All rights reserved.