Cell culture longevity in fed-batch culture of hybridomas is often limited by elevated medium osmolality caused by repeated nutrient feeding. Shotwise feeding of 10ร Dulbecco's modified Eagle's medium (DMEM) concentrates elevated the osmolality of medium up to 540 mOsm/kg at the end of fed-batch culture of S3H5/โฅ2bA2 hybridoma which is known to be lethal to most hybridomas. S3H5/โฅ2bA2 hybridoma has been shown to grow without significant growth depression at 219 mOsm/kg in DMEM supplemented with 10% fetal bovine serum. To improve culture longevity in fed-batch cultures of S3H5/โฅ2bA2 hybridoma, a hypoosmolar medium (223 mOsm/kg) was used as an initial basal medium. The use of hypoosmolar medium delayed the onset of severe cell death resulting from elevated osmolality and allowed one more addition of 10ร DMEM concentrates to the culture. As a result, a final antibody concentration obtained was 121.5 ยตg/mL which is approximately 1.5-fold higher compared to fed-batch culture using a standard medium (335 mOsm/kg). When compared to batch culture, a more than 5-fold increase in the final antibody concentration was achieved. Taken together, the use of hypoosmolar medium as an initial medium in fed-batch culture improved culture longevity of S3H5/โฅ2bA2 hybridoma, resulting in a substantial increase in the final antibody concentration.