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The concentration of glutamine and/or asparagine in your cell culture media should be assessed and limited since both amino acids can be deaminated leading to ammonia as the byproduct. During late stationary phase of mammalian cell culture, cells may utilize alanine to generate pyruvate and this process also produces ammonia. Supplementation of pyruvate into the culture is a possible solution to alleviating ammonia accumulation. Additionally, the issue can potentially be addressed from a process perspective by implementing a temperature shift scheme or adjusting the harvest time.