When mouse hybridoma cells were grown in culture media which were made hyperosmotic through the addition of NaCl or sucrose, the specific rate of antibody production increased with medium osmolality, reaching approx. 1.9 times the level obtained at physiological osmolality. However, due to a simultaneous reduction of the maximal cell density in the hyperosmotic media, the effect of the increased production rate did not give significant increases in the maximum antibody titer obtained in the cultures. When the osmoprotective compound, glycine betaine, was included in the NaCl- or sucrose-stressed cultures, the specific antibody production rate was increased up to 2.6-fold and maximum antibody titer up to twofold over that obtained in the control culture (physiological osmolality). A similar pattern of response was observed when other osmoprotective compounds (sarcosine, proline, glycine) were added to NaCl-stressed hybridoma cell cultures. For the present experiments, the results suggest that medium osmolality, rather than growth rate, will determine the specific antibody production rate by hybridoma cell line 6H11 growing in hyperosmotic culture media.