The growth yields for glucose and glutamine of murine hybridoma cells entrapped in collagen gel particles were examined during the growth phase. The immobilized hybridoma cells were cultivated in a fluidized bed fermenter where the medium was circulating to supply oxygen separately. Procedures to supply an alkaline solution for adjusting the pH level strongly affected the growth yields. A direct supply of the alkaline solution to the cultivation system reduced both the growth yields for glucose and glutamine, probably due to a local increase in pH level. On the other hand, when fresh medium in which the pH was adjusted to around 8.5 was added to the cultivation system, the growth yields were unchanged even at the same pH level as when direct alkaline supply was used. These results suggest that an indirect alkaline supply could be recommended to adjust the pH level when using medium-circulating-fermenters.