1. The incidence of diarrhoea, digestibilities of nutrients and the faecal bacterial flora were compared among three groups of Holstein male calves up to 3 weeks of age. Two groups of four calves were given a milk-substitute containing tallow by nipple-pail (group TN) and by open-bucket (group TB). The third group of four calves was nipple-fed a milk-substitute containing soya-bean oil (group SN). Each of the milk-substitutes contained approximately 300 g milk-protein and 100 g fat/kg dry matter (DM).
2. Mean faecal DM contents (g/kg) were 217, 185 and 112 for groups TN, TB and SN respectively and the corresponding pH values were 7·21, 7·00 and 6·50. The difference between groups TN and SN was statistically significant (P < 0·05).
3. No difference was observed between groups TN and SN in the apparent digestibilities of DM, crude protein (CP; nitrogen × 6·25), diethyl ether extract (EE) and total reducing sugars. But in the group TB, the digestibility of EE was significantly lower (P < 0·05), and that of CP tended to be, though not significantly, lower than in the other two groups.
4. Bacterial flora in faeces showed considerably wide quantitative variations among individual calves, but there was a tendency for increased viable counts of Lactobacilli in faeces of group SN.
5. The present results suggested that an appreciable difference in the mechanism would exist between diarrhoea occurring when milk-substitute was offered by bucket and when highly-unsaturated vegetable oils were contained in it. Possible mechanisms were also discussed.