The effect of the immunomodulator Natstim ® (lyophilised killed bacterial cells from Escherichia coli 29, Escherichia coli J5, Staphylococus aureus 27/58 at amounts corresponding to 10 10 cells of each species, stabilized with Dextrane 40) on growth performance and two factors of natural humoral immunity serum lysozyme concentrations and the activity of the alternative pathway of complement activation (APCA) was investigated. The results for the experimental group showed a tendency towards higher live weight at 35 and 49 days of age.