In the present study, the steady-state cell density (X) of chemostat cultures of murine hybridoma was varied by the concentration of glucose and glutamine in culture medium and the dissolved oxygen partial pressure. Except at low glutamine and low oxygen levels, the specific death rate (kd) of the cultures was found to decrease with increasing dilution rate (D). However, the plot of kd vs. X/D yielded linear relation, which suggests that cell death was due to a non-growth-linked inhibitory product of the cells. The kd value measured at low glutamine and low oxygen levels remained practically unchanged over a wide range of D between 0.020 and 0.029 h−1. The kd for low oxygen cultures was always lower than the values obtained in low glucose and low glutamine cultures. A low-molecular-weight component of possibly less than 3000 MW was detected to be cell-death-inducing in the supernatant of exponentially growing cultures. It was neither lactate nor ammonium. The autoinhibitor was not cell-line specific. © 1995 John Wiley & Sons, Inc.