The effect on diarrhoeal disease of an acidified, modified powdered cow's milk infant formula (Pelargon) was evaluated in 82 infants (Group I) for six months; 104 infants who received the same formula but non acidified, served as controls (Group II). Nutritional status remained satisfactory in both groups throughout the observation period. Some children rejected the taste of the acidified milk. The incidence of diarrhoea was lower in Group I (p less than 0.001). The proportion of days in which the children suffered from acute diarrhoea, and the duration of the episodes were also lower in the children given the acidified milk (p less than 0.001). The rate of detection of enteropathogens and the species identified were comparable in both groups. Carrier rates for bacterial enteropathogens fell over time in Group I while they rose in Group II (p less than 0.001). Carrier rates for enteric parasites were comparable to those expected in our setting for this age group. These results suggest that acidified milk exerts a protective effect against diarrhoeal disease.