A study has been made of the effects of a combination of lactoperoxidase system (LP-s) and lactoferrin (LF), supplemented to milk replacer for 2 weeks, on the morphology of the small intestinal mucosa, microbiology of the intestinal tract and the incidence and severity of diarrhoea in young calves. The experiment was conducted in two groups of 15 young calves, aged 7 to 21 days. The results showed a lower incidence and severity of diarrhoea (P < 0.05) in the calves of the LP-s/LF group compared to the control group. CFU (Colony forming units) of E. coli in colonic digesta (P = 0.10) and in faeces (P < 0.05) were significantly lower in the LP-s/LF group compared to the control group. Dissecting microscopy of the small intestinal mucosa indicated more finger-shaped villi in the distal jejunum of the calves of the LP-s/LF group compared to the control group (P = 0.01). Histometrical measurements showed that these villi were significantly (P = 0.002) higher. The mean contents of immunoglobulins (IgG) increased (P = 0.1) during the test period whereas those of the control group remained unchanged.