The hemagglutinin of influenza virus (HA), an acid-activated membrane fusion protein, is synthesized in the endoplasmic reticulum and transported through the Golgi complex to the cell surface of infected cells as an uncleaved, fusion-incompetent precursor, HA0. The mature, proteolytically activated HA is known to undergo a rapid, irreversible, acid-induced conformational change which mediates membrane fusion and virus penetration. On the basis of antigenic modifications and the acquisition of trypsin susceptibility, we demonstrate here that HA0, while unable to cause fusion, is acid sensitive. It undergoes irreversible conformational changes quite similar to those of HA at mildly acidic pH (pH less than 6.0). The ectodomain of HA0 does not, however, acquire hydrophobic properties and the changes occur in a less concerted manner (the pH dependence is much broader and the rate of conversion slower). These differences are likely to account for the inability of acid-treated HA0 to trigger membrane fusion. It was shown, moreover, that HA0 acquired its acid-sensitive properties immediately following trimerization in the endoplasmic reticulum. Since HA0 did not convert to the acid form at any point during its intracellular transport, we concluded that the trans-Golgi compartment, known to be more acidic than the cytosol and involved in constitutive membrane transport, is not likely to have a pH less than 6.0.