The effect of cloned gene dosage on growth and product formation under hyperosmotic conditions has been studied using recombinant Chinese hamster ovary (rCHO) cell lines producing chimeric antibody. Batch cultures of four rCHO cell lines carrying different numbers of antibody gene copies were carried out using the hyperosmolar medium. Depending on cloned gene dosage, hyperosmotic pressure decreased specific growth rate (mu) and increased specific antibody productivity (q(Ab)) to a different degree. The cell line with lower cloned gene dosage displayed more significant enhancement in q(Ab) and less reduction in mu at hyperosmolalities. However, the cell line with higher cloned gene dosage still yielded higher maximum antibody concentration at hyperosmolality up to 469 mOsm/kg. Northern blot analysis showed a positive relationship between immunoglobulin mRNA level per cell and q(Ab), indicating that transcriptional regulation was involved in the response of rCHO cells to hyperosmotic pressure. Cell cycle analysis showed that hyperosmotic pressure induced G(1)-phase arrest, suggesting that the increase of cell population in G(1)-phase may contribute in part to enhanced q(Ab) at hyperosmolality. Taken together, although the cell line with lower cloned gene dosage displayed more significant enhancement in q(Ab) at hyperosmolality, the factor that determined the maximum antibody concentration in hyperosmotic rCHO cell cultures was almost exclusively the gene dosage.