The stability of ampicillin, piperacillin and cefotaxime, alone or in combination with either netilmicin or amikacin, was tested by microbiological methods at 29 degrees C (ampicillin, also at 22 degrees C) in an L-amino acid solution specially prepared for newborn infants. In the case of ampicillin, the mean concentration had dropped to 84% of the initial value after 24 hours. This fall is acceptable. Piperacillin and cefotaxime were stable. Admixture of netilmicin or amikacin did not affect the activity of ampicillin, piperacillin and cefotaxime. The concentrations of netilmicin and amikacin were stable for 24 hours. Combination with ampicillin, piperacillin or cefotaxime did not affect the activity of either aminoglycoside. The antibiotics investigated could therefore be used in our TPN infusion system.