The effects of L. johnsonii on gut microflora, bird performance and intestinal development were assessed using 288 day-old Cobb broilers challenged with Salmonella sofia (S. sofia). The experiment was a 3 x 2 factorial design which consisted of three treatments, a negative control (NC) with no additives, a positive control (PC) containing antimicrobials (zinc-bacitracin, 50 ppm) and a probiotic group (Pro), and with the two factors being unchallenged or challenged with S. sofia. A probiotic preparation of L. johnsonii (109 cfu/chick) was administered to chicks individually by oral gavage on days 1, 3, 7 and 12. Chicks were individually challenged with Salmonella sofia (107 cfu/chick) by oral gavage on days 2, 8 and 13. Results showed that the challenge itself markedly reduced (P