Hybridoma cells, S3H5/γ2bA2, were grown in spinner flasks containing RPMI 1640 media at different levels of serum. Cells in low serum media (1% fetal bovine serum) were more sensitive to shear induced by mechanical agitation than cells in high serum media (10%), indicating that serum can protect cells from shear induced by mechanical agitation. The data suggest that serum alters the physiological properties of the cells to make them more shear-resistant, rather than by changing the physico-chemical properties of medium.