The effect of a well-defined mild shear stress on hybridoma cell viability (HB-8852) in a serum-free culture medium has been analysed, and the role as shear protector of different concentrations of fetal bovine serum have been studied. Samples harvested from cultures in their late exponential growth phase, were subjected in a rheometer to a constant shear stress of 0.41 +/- 0.02 Pa, and the evolution of viable and total cell concentrations was determined and compared with static controls. A simple segregated kinetic model for the viable and dead cells was used to know the effect of serum concentration on the specific cell growth and death rate of the cells.