Yogurt has been shown to inhibit, by various mechanisms, pathogenic bacterial growth in vitro, including that of the rabbit Escherichia coli strain RDEC-1. To determine whether this in vitro inhibition by yogurt has an in vivo counterpart, 60 newly weaned New Zealand rabbits were randomly assigned to receive a diet supplemented with either milk or yogurt. Four days later, rabbits were infected intragastrically with 10(8) E. coli strain RDEC-1. During the 30 days postinfection, animals were checked regularly for weight, stool characteristics, and fecal excretion of bacteria. Results show that the duration of diarrhea as well as bacterial excretion were the same in the milk and yogurt groups, indicating that in vivo, yogurt did not interfere with the growth of the pathogenic E. coli strain RDEC-1. The pattern of the survival curves was significantly different in the two groups (p less than 0.03), but the difference between their mortality at 30 days postinfection was not significant (80% in the milk group versus 67% in the yogurt group). These results indicate that the bactericidal activity of yogurt observed in vitro against the E. coli strain RDEC-1 does not have an in vivo counterpart in the model of the just-weaned rabbit infected by RDEC-1.