Bifidobacteria, lactobacilli and staphylococci are the predominant organisms in the faeces of breast fed babies whereas in formula fed babies coliforms, enterococci and bacteroides predominate. In vitro studies suggest that the mechanisms responsible are probably related to the acid base properties of the formula and 'immunological' proteins such as lactoferrin and sIgA. In human babies however the addition of bovine lactoferrin to an infant formula has little effect on the faecal flora and does not move it in the direction of the breast fed baby. There are various possible explanations of this lack of effect, e.g., inactivation of the lactoferrin when it is added to a formula, and immunological responses in the intestine to a foreign protein. We consider the most likely explanation is that other factors necessary for the optimum activity of lactoferrin were not present or in inappropriate concentration, e.g. sIgA, lysozyme, citrate, bicarbonate. If human lactoferrin is added to an infant formula it may be these other factors will require attention if the lactoferrin is to have a significant effect. An iconoclastic interpretation which cannot be completely excluded is that the hypothesis of lactoferrin bacteriostatic activity is based on in vitro studies and is not a reflection of what happens in babies. The faecal flora of a breast fed baby is very different from that of a baby receiving either cow's milk or a modern infant formula (Figure 1). Among breast fed babies bifidobacteria lactobacilli and staphylococci are the predominant organisms, whereas in formula fed babies the predominant organisms are enterococci, coliforms, and bacteroides (1).