The compatibility of 28 parenteral antibiotics with a pediatric parenteral nutrition solution was tested using a piggyback injection technique. The parenteral nutrition solution, apparatus, and antibiotic doses simulated central venous administration to 5- and 30-kg patients. All antibiotics were injected into a running parenteral nutrition solution, distal to an in-line filter. After passing through a central venous catheter, immersed in a heated water bath, the effluent was collected, analyzed for pH, and visually inspected for precipitate formation. The effluent was not evaluated for the presence of microscopic precipitates. When given as piggyback injections, five of the 28 antibiotics increased the pH of the original parenteral nutrition solution. These were ampicillin, cefamandole, cephalothin, cephradine, and oxacillin. Two of these, ampicillin and cephradine, produced a heavy visible precipitate, found to consist of calcium and phosphorus. The injections of certain antibiotics promote the formation of an insoluble calcium phosphate precipitate. When calcium and phosphate are present in a parenteral nutrition solution, a flush procedure is recommended when injecting antibiotics which increase the pH of the original parenteral nutrition solution.