Secretory immunoglobulin A (SIgA) plays an important role in the defence of the gastrointestinal tract. The level of faecal SIgA antibody is associated with increased neutralization and clearance of viruses. Formula-fed infants who lack the transfer of protective maternal SIgA from breast milk may benefit from strategies to support maturation of humoral immunity and endogenous production of SIgA. We aimed at studying the effects of standard, prebiotic and probiotic infant formulas on the faecal SIgA levels. At birth, infants of whom the mother had decided not to breastfeed were allocated to one of three formula groups in a randomized, double-blind fashion. Nineteen infants received standard infant formula; 19 received prebiotic formula containing a specific mixture of 0.6 g galacto-oligosaccharides (GOS)/fructo-oligosaccharides (FOS)/100 ml formula and 19 received probiotic formula containing 6.0 x 10(9) cfu Bifidobacterium animalis/100 ml formula. Faecal samples were taken on postnatal day 5, day 10, wk 4 and every 4 wk thereafter until wk 32. SIgA in faeces was determined by an enzyme-linked immunosorbent assay. During the intervention, infants fed on prebiotic formula showed a trend towards higher faecal SIgA levels compared with the standard formula-fed infants reaching statistical significance at the age of 16 wk. In contrast, infants fed on the probiotic formula showed a highly variable faecal SIgA concentration with no statistically significant differences compared with the standard formula group. Formula-fed infants may benefit from infant formulas containing a prebiotic mixture of GOS and FOS because of the observed clear tendency to increase faecal SIgA secretion. Adding viable B. animalis strain Bb-12 to infant formula did not reveal any sign for such a trend.