AbstractThe nature and temporal development of ammonia inhbition were investigated in batch, fed‐batch, and continuous cultures. Significant inhibition was observed when cells were inoculated in serum‐containing or chemically defined medium containing more than 2 mM of ammonia. In contrast, no inhibition was observed at greater than 10 mM when the ammonia concentration was gradually increased over the span of a batch culture by feeding ammonium chloride. Strong growth inhibition was observed after each of five step changes (2.8 → 3.7 → 4.0 → 4.9 → 7.7 → 13.5 mM) in continuous culture. Following a period of adaptation at each higher value, the viable cell density stabilized at a new lower value. The lowering in viable cell density was caused by an increase in specific death rate and a decreased cell yield on glucose, glutamine, and oxygen. Increased ammonia concentration had little or no effect on the steady‐state specific growth kinetics or specific antibody productivity. © 1994 John Wiley & Sons, Inc.