The protective effect of Enterococcus faecium EF 55 against Salmonella enterica serovar Enteritidis phage type 4 (SE PT4) was studied in 1-day-old chicks. The EF 55 strain (isolated and characterised by the authors earlier) was applied daily (1.10(9) CFU/0.2 ml PBS) for 7 days. Oral inoculation of the SE PT4 strain was performed on day 8 in a single dose of 5.10(8) CFU/0.2 ml PBS. The experiment lasted for 21 days. Samples were collected on day 1 of the experiment to verify the absence of Salmonella, on day 8 to check colonisation of EF 55 and immunological status in experimental birds, and on days 2, 4, 6, 8 and 14 after SE PT4 infection of chicks. Strain EF 55 sufficiently colonised the digestive tract of chicks after 7 days of application. The highest numbers of EF 55 in the faeces of chicks were observed before SE infection and persisted to day 6 post infection (p.i.) in both the EF and EF+SE groups. PCR confirmed the identity of the EF 55 strain. The counts of SE PT4 strain in faeces of the EF+SE group were significantly reduced in comparison to those in the SE group on days 2 and 14 p.i. (P < 0.01). The significant reduction of salmonellae in the caecum was recorded at the end of the experiment (day 14 p.i.) in the EF+SE group in comparison to the SE group (P < 0.01). At day 4 p.i., colonies of S. Enteritidis PT4 were found in the liver of chicks of the SE group in a higher concentration than in chicks of the EF+SE group (P < 0.001). Salmonellae were isolated from the liver until days 8 and 6 p.i. in the SE and EF+SE groups, respectively. The mean values of actual lymphocyte subpopulations in the blood and the relative percentage of caecal intraepithelial lymphocyte subpopulations (CD4, CD8, CD44, TCR, MHC II and IgM) were not influenced at a statistically significant level by the application of the EF 55 and/or the SE PT4 strain. The results demonstrate the antimicrobial effect of E. faecium EF 55 against S. Enteritidis PT4.