Fat absorption of an adapted cow's milk formula was studied in a randomized controlled trial involving two groups of 18 premature infants (mean gestational age +/- SD: 33.0 +/- 2.9 weeks, range 26.5-37.5 weeks). The triglyceride configuration was modified by the use of lard. This modification did not improve the absorption of fat or energy. Also no difference in serum concentrations of cholesterol and triglycerides was found. Growth velocity during the study was similar in both groups. Detailed analysis of the data revealed that in infants who received (parenterally) antibiotics (mainly ampicillin and netilmicin) a higher coefficient of fat absorption (+20%, P less than 0.01) and of energy absorption (+8%, P = 0.03) was found. Based on these results, we find no support for the use of lard in adapted cow's milk infant formulas to improve fat absorption. In studies of fat and energy absorption the effects of antibiotics have to be taken into account.