Terminal sialylation of therapeutic glycoprotein is important for biological activity and in vivo stability. The enzyme $alpha$2,3-sialyltransferase is the key enzyme that links sialic acids to the termini of glycans in the Chinese hamster ovary (CHO) cell line. Terminal sialylation is affected by numerous factors, but the elements that regulate $alpha$2,3-sialyltransferase are not known. We investigated the relationship between $alpha$2,3-sialyltransferase activity, ammonium concentration, and cell attachment area-based cell concentration in a recombinant human erythropoietin (rhEPO)-producing CHO cell line. We found that ammonium in the culture medium had almost no effect on $alpha$2,3-sialyltransferase activity, but that the activity was affected by cell attachment area-based cell concentration; $alpha$2,3-sialyltransferase activity and terminal sialylation of rhEPO decreased with increasing the cell concentration. These results demonstrate that the cell attachment area-based cell concentration is an important factor that affects 2,3-sialyltransferase activity and terminal sialylation of CHO cells.