The effects of treatment/prophylaxis of newborn calf colibacillosis with tryptic casein hydrolysate (TCH), recently shown to be a novel type of antimicrobial acting through stimulation of the microbial autolytic system, versus an authorized veterinary drug, Fermosorb, were evaluated. Both products showed similar high therapeutic and prophylactic efficacies, but hematological indices and daily weight gain of cured/protected animals were better with TCH. The differences in hemoglobin and hematocrit levels, total protein, gamma-globulin and sulfhydryl group quantities, bactericidal and lysozyme activities as well as daily weight gain at the end of treatment/prophylaxis were statistically significant (P<0.05-0.000005). Statistically significant differences (P<0.05-0.0005) in favor of TCH were also observed when bactericidal activity, total protein quantity of serum as well as daily weight gain of the animals were compared on the 90th day after birth. We conclude that TCH acts not only as an antimicrobial, but also as an immunostimulant (and growth promoter). The immunostimulatory activity of TCH most probably derives from a synergistic action of bioactive peptides encrypted in the preparation itself and the cell wall fragments resulting from microbial autolysis induction.