The predominance of taurine (Tau) conjugated over glycine conjugated bile acids in infants fed human milk as opposed to those on formulas without added Tau could account for a more complete absorption of fat. Fifteen low birth weight infants were randomized to either Enfamil Premature or to Enfamil Premature added with 40 mumol/dl of Tau and compared to a third group made up of nine low birth weight infants fed their own mother's preterm milk. Formulas and human milk were fed according to tolerance and constituted the sole nutrition for 3 months. A metabolic study was carried out at 3 wk of age and control of growth was done periodically. Urinary Tau excretion (mumol/dl) was very low (p less than 0.001) in the group fed Enfamil Premature (0.3 +/- 0.1) when compared to the values obtained in infants supplemented with Tau (51.6 +/- 12.5) and in those on human milk (36.3 +/- 7.9). Infants supplemented with Tau (92.5 +/- 1.2) had a coefficient of fat absorption which was higher (p less than 0.05) than the unsupplemented group (87.5 +/- 7.9) and comparable to the human milk-fed group (91.6 +/- 1.4). The effect was more pronounced on the saturated fatty acids and varied inversely with their individual water solubility. There was no effect of Tau on nitrogen retention and growth was identical in the three groups. These data show that the addition of Tau to formula had no effect on growth but improved the absorption of fat especially saturated fatty acids which require higher concentrations of bile acids to form mixed micelles.