Thirteen newborn infants, 8 term and 5 preterm (gestational age 31 to 36 weeks), were treated for between 3 and 7 days with gentamicin and ampicillin or cloxacillin because of suspected bacterial infection. The dosage of gentamicin was carefully monitored by serum concentration assays. Urinary alanine aminopeptidase, urinary beta 2-microglobulin, serum urea, and serum beta 2-microglobulin were measured during and after the end of treatment to detect signs of renal toxicity. Levels of urinary aminopeptidase increased in 12 of them, indicating damage to the cells of the proximal tubuli. Changes in urinary beta 2-microglobulin followed the normal physiological course seen in neonates after birth. Serum levels of urea and beta 2-microglobulin did not indicate any drug-associated depression of glomerular filtration rate.