Calves fed whole milk had 2,000-fold fewer (P less than 0.001) coliforms in the cranial part of the small intestine than did calves fed skim milk (fat removed). Calves fed milk with 32 mM added propionic acid had nearly 1,000-fold lower (P less than 0.001) counts of lactobacilli in the entire gastrointestinal tract than did calves fed milk without added propionic acid.