Balance studies were conducted on preterm infants (birthweight, 1,500 g or less) fed their own mother's milk or formula (SMA with iron) to compare the relative adequacy of these sources for copper (Cu), iron (Fe), and zinc (Zn). Urine and stools were collected for 72 h from infants aged 1, 2, or 4 weeks while they were fed milk or formula. Infants fed SMA 24 (but not SMA 20) received Cu, Fe, and Zn intakes within the recommended range. They did not, however, achieve estimated in utero retention rates for Fe, and were in negative Cu balance; they did approach in utero retention rates for Zn. Infants fed their own mother's milk received Cu and Zn, but not Fe, in recommended amounts. They achieved in utero retention rates for Cu at each age studied and approached the estimated in utero retention rates for Zn at 4 weeks, but they retained insufficient Fe throughout. Thus, neither the infant's own mother's milk nor this particular formula provides ideal amounts of all three minerals studied--Cu, Zn, and Fe--during the 1st month of life.