The effect of cloned gene copy number on growth and product formation has been studied in sufficient detail using a Chinese hamster overy (CHO) cell line producing recombinant hepatits B surface antigen (HbsAg). Batch culture experiments were carried out in T flasks in order to characterize cell growth and HbsAg secretion in various clones carrying different numbers of HbsAg gene copies integrated into CHO cell chromosomes. Specific growth rates were found to decrease with increasing gane copy number. Secreted HbsAg concentration and specific HbsAg secretion rates were found to increase with increase in gene copy number. Gene copy numbers in each clone determined using Southern hybridizations were positively correlated with intracellular dihydrofolate reductase (dhfr) content using a flow cytometric assay. The mRNA levels quantitated using Northern hybridization followed by autroadiography and densitometry also gave the same trends. The flow cytometry experiments show that while parental cells were quite homogeneous with respect to intracellular dhfr content, the amplified clones exhibit a great deal of heterogeneity in dhfr content. Pulse-chase experiments show that the efficiency of HbsAg secretion (defined here as the fraction of initially labeled HbsAg that is secreted into the extracellular medium at the end of a 23.5-h chase) decreases and also that the intracellular HbsAg degradation increases with increasing gene copy number.