Day-of-hatch Leghorn chicks were treated by oral gavage with PREEMPT, a continuous-flow competitive exclusion culture containing broiler caecal bacteria, followed by an oral challenge with Listeria monocytogenes, to determine the effects of PREEMPT on L. monocytogenes caecal colonization. Increased (P < 0.001) concentrations of caecal propionic acid in control chicks compared with PREEMPT-treated chicks at 3 days of age were indicative of the establishment of the PREEMPT bacteria. Caeca from control chicks at 7 days after the oral challenge with L. monocytogenes contained mean 3.4 +/- 1.4 log cfu g-1 of caecal content, while caeca from PREEMPT-treated chicks contained no detectable Listeria. Enrichment for L. monocytogenes resulted in 100% of caeca from control chicks testing culture-positive for L. monocytogenes, while none of the caeca from PREEMPT-treated chicks were culture-positive. The results indicated that prophylactic treatment of newly hatched chicks with PREEMPT significantly reduced caecal colonization by L. monocytogenes.