The competitive exclusion (CE) product (Broilact) which is effective against Salmonellas, was found to be inactive against campylobacters. Microecological concepts were applied in the search of a new competitive flora and two novel strains ('K-bacteria') were isolated. These strains resembled campylobacters but differed from them in morphology, enzyme profiles (API), cellular fatty acid profiles and when tested with a ribosomal RNA hybridization probe (Gene-Trak). Two-week laboratory trials on broiler chickens showed that CE treatment may protect the birds against campylobacters but revealed the need for facultatively anaerobic bacteria in establishing a protective flora. A 5-week pilot scale trial was carried out. The trial involved 1800 newly hatched chicks in 30 groups. K-bacteria and Broilact, which provided the necessary facultatively anaerobic bacteria, were administered to some of the birds in the first drinking water. A seeder bird technique was used to challenge experimental and control birds with Campylobacter jejuni biotype 2 (broiler origin). Three seeder birds were placed in each group of 60 birds. Groups were sampled weekly for campylobacters and finally at the slaughterhouse. From each group, the caecal contents of two birds were examined quantitatively for campylobacters. The performance of the birds was also monitored during the trial. The results showed a 1.5 week delay in the onset of campylobacter infection in treated chicks and a consistently lower level of colonization in comparison with control birds. At slaughter, levels of carriage in caecal contents of treated birds were 1.5-2.0 log10 units lower than those of controls, despite apparent stress from harvesting and transportation. The treatment had no economically important effects on the performance of the birds during rearing.