The secretion rate of activated protein C (APC) by BHK cells was increased 35-fold by increasing the cDNA copy number per cell from 50 to 240. In this range, the relation between APC secretion and cDNA copy number was not linear and the rate of APC secretion per cDNA copy increased sevenfold. This apparent cooperative effect of multiple cDNA copies could be related to their integration in tandem. For cDNA copy numbers higher than 240, the APC secretion rate per cDNA and per cell decreased dramatically. The gamma-carboxylation of glutamic acid residues, a posttranslational modification required for APC biological activity, was also investigated. The proportion of APC that was fully gamma-carboxylated decreased as the secretion rate of APC increased.