In the summer of 2010, parenteral nutrition (PN) admixtures were administered to neonates in the Pediatric Department of the University Medical Center Mainz that provoked severe clinical sequelae. Contamination of a dummy infusion with Enterobacter cloacae and Escherichia hermannii was detected on the day of the incident, and the same isolates were subsequently grown from all PN admixtures as well as from the parent amino acid solution from which the admixtures had been prepared. Quantitative microbiological analyses paired with the determination of endotoxin concentrations enabled the conclusion to be reached that the amino acid solution had represented the primary source of contamination, which must have occurred in the distant past and may have derived from passage of the bacteria through a crack in the glass container. The findings have large implications, and the approaches employed should become of value when similar incidents occur again in the future.