Ammonia is a major concern in biotechnology because it often limits recombinant protein production by animal cells. Conditions, such as ammonia accumulation, in large-scale production systems can parallel those that develop within fast-growing solid tumors such as small cell lung cancer (SCLC). Ammonia's specific inhibition of the sialylation of secreted glycoproteins is well documented, but it is not known how ammonia affects membrane-bound proteins, nor what role it may have on important glycosylation determinants in cancer. We therefore examined the effects of NH4Cl on polysialic acid (PolySia) in the neural cell adhesion molecule (NCAM). By using flow cytometry combined with two NCAM antibodies, one specific for the peptide backbone and another that recognizes PolySia chains, we show that ammonia causes rapid, dose-dependent, and reversible inhibition of NCAM polysialylation in Chinese hamster ovary (CHO) and SCLC NCI-N417 cells. The decrease in PolySia was accompanied by a small increase in NCAM, suggesting that the changes were specific to the oligosaccharide. Inhibition by ammonia was greater for CHO cells, with PolySia cell surface content decreasing to 10% of control after a 4-day culture with 10 mM NH4Cl, while N417 cell PolySia was reduced by only 35%. Ammonia caused a 60% decrease in the CHO cell yield from glucose, while N417 cells were barely affected, suggesting that increased resistance to ammonia by N41 7 cells is a global rather than glycosylation-specific phenomenon. The data presented show that the tumor microenvironment may be an important factor in the regulation of PolySia expression.