alpha 1-Microglobulin (alpha 1-m, protein HC), a relatively low molecular weight protein of about 31,000 daltons, was measured in urine of three groups of 34 preterm neonates: group A consisted of 9 healthy preterm neonates; groups B (n = 13) and C (n = 12) consisted of preterm neonates with suspected or confirmed bacterial infections. Immediately after birth, all group B neonates were treated with ampicillin and aztreonam in combination, and all group C neonates were treated with oxacillin and amikacin in combination. To optimize amikacin administration, computerized individually tailored doses were administered. Urine samples were obtained from a short collection in sterile bags on the 1st, 4th, and 7th day after delivery in all infants. Urinary alpha 1-m concentrations were measured by a turbidimetric method (latex agglutination photometric immunoassay) and results were expressed as a ratio to urinary creatinine. In group A, urinary alpha 1-m concentrations were stable after birth. In group C, alpha 1-m excretion increased immediately within the 1st day of treatment, and over the 1st week of life urinary alpha 1-m levels were significantly higher than in group A (P = 0.033). These data support the conclusion that amikacin administration was the most important factor inducing renal tubular dysfunction in the neonates of group C.