The amino acid L-glutamine has been shown to be important to numerous cells in the body. However, the routine measurement of glutamine in biological solutions is complicated by its relative instability, particularly at extremes of pH. We report a simple bacterial bioassay method for the estimation of glutamine, carried out at neutral pH, using an Escherichia coli that is specifically dependent on the presence of glutamine for replication. The assay is simple to perform and we have shown it to be highly reproducible. Furthermore, we observed significant correlation between the estimations of glutamine by the bioassay system and current methodologies. The levels of glutamine recorded were higher than by other methods. We also found that deproteinization and neutralization of samples allowed them to be stored at -70 degrees C for up to 24 weeks without deterioration.