Ascorbic acid stability in TPN infusions in 3-litre plastic bags was examined. Vitamin C was found to degrade slowly in mixtures which do not contain trace elements. In the presence of copper, degradation proceeds rapidly until dissolved oxygen is depleted. Reducing the copper concentration had only a minor influence on degradation rate. However, this copper-catalyzed reaction was prevented if cysteine was present in the TPN regimen. The amount of ascorbic acid degraded depended on the dissolved oxygen content of the infusion, the amount of residual air in the bag after filling and the permeability of the plastic to oxygen. In the absence of copper, 20–30 mg ascorbic acid was broken down within 24 h at ambient temperatures, but if copper was present, 150–200 mg was degraded within 2–4 h. The contribution of dehydroascorbic acid to the amount of vitamin C delivered to the patient was negligible. It is concluded that either vitamin C and trace element injections containing copper should not be added to the same bag, or an adequate coverage of ascorbic acid must be included to allow for losses by oxidation before and during administration.