To investigate the effects of lactate on cell growth and antibody production, a new method of maintaining the lactate concentration constant in a fed-batch culture is described. When the pH was initially adjusted by sodium hydroxide, the specific growth rate decreased and specific death rate increased with an increase of lactate concentration. To investigate whether the inhibition was due to the lactate concentration itself or to the osmotic pressure, the effect of the osmotic pressure adjusted by sodium chloride was compared with that of sodium lactate. When the osmotic pressure was adjusted to same condition as that of sodium lactate using sodium chloride, the specific growth data showed the same degree of growth inhibition. It was thus evident that the inhibition to cell growth was mainly due to osmotic pressure while lactate production from glucose was found to be inhibited by the lactate itself compared with sodium chloride. The specific antibody production rate had a maximum value within a certain range of lactate concentration. Moreover, specific antibody production rate had a unified relationship with the kinetic parameter mu, in spite of the different causes of inhibition by lithium lactate and sodium lactate. A certain "trade-off" relationship between growth and antibody production existed at higher growth rates.