Lactate and ammonia are the two major waste products formed during mammalian cell growth. Accumulation of these side products can have a negative effect on cell growth, and has drawn recent attention because of their inhibitory effects on the specific product synthesis rate. Our aim is to reduce lactate formation in the cell culture by genetically manipulating of the pathway of lactate synthesis with an aim to achieve high monoclonal antibody production. We have partially disrupted the LDH-A gene by homologous recombination in hybridoma cells (ATCC-CRL-1606). The cells that received the newly introduced DNA were selected by G418, and an LDH-deficient cell was identified by a screening method based on medium color changing in 96-well plates. A variant cell, LDH-neo21, was identified through this screening method and was characterized. The specific productivity of lactate by LDH-neo21 cells was 50% lower than that of parental cells. Intracellular LDH enzyme activity was significantly reduced. The cell growth was improved both in terms of cell density and cell viability. Total cell density potentially reached 5 x 10(6) cells/mL while the parental hybridoma cells had a cell density of 3.5 x 10(6) cells/mL, which represented a 30% increase. The antibody production of LDH-neo21 cells was threefold greater than that of parental cells during 5-day batch culture. Polymerase chain reaction (PCR) results showed that at least one copy of the LDH-A gene was disrupted in the LDH-neo21 cells. The variant of the hybridoma cell exhibited a significant advantage of reduced lactate formation in the cell culture with a high concentration of glucose, which led to a higher production of monoclonal antibody. 2001 John Wiley & Sons, Inc.