The folding of influenza hemagglutinin (HA0) in the ER was analyzed in tissue culture cells by following the formation of intrachain disulfides after short (1 min) radioactive pulses. While some disulfide bonds were already formed on the nascent chains, the subunits acquired their final disulfide composition and antigenic epitopes posttranslationally. Two posttranslational folding intermediates were identified. In CHO cells constitutively expressing HA0, mature HA0 subunits were formed with a half time of 3 min and their folding reached completion at 22 min. The rate of folding was highly dependent on cell type and expression system, and thus regulated by factors other than the sequence of the protein alone. Exposure of cells to stress conditions increased the level of glucose regulated proteins, including BiP, and decreased the folding rate. The efficiency of folding and subsequent trimerization was not dependent on the rate of translation, nor on temperature between 37 and 15 degrees C; however, the rates of folding and trimerization decreased with decreasing temperature. Whereas the rate of folding was independent of expression level, trimerization was accelerated at higher levels of expression.