We have characterized the physiological defect in two temperature-sensitive mutants of the WSN strain of influenza virus which possessed a lesion in the haemagglutinin (HA) gene. In mutant virus-infected cells at the non-permissive temperature, the precursor HA polypeptide containing predominantly mannose-rich carbohydrate chains was not converted to the mature, functional HA polypeptide. Immunofluorescence showed that the HA polypeptide did not appear on the cell surface but was confined largely to the Golgi apparatus. It was concluded that the major physiological defect of these mutants was a block in the transport of the HA polypeptide beyond the Golgi apparatus. The block could be reversed, however, by lowering the temperature to 34 degrees C, resulting in normal processing of the precursor polypeptide and emergence of infectious progeny virus within 30 min. The HA activity of the two mutants, but not wild-type virus, was rapidly inactivated at 51 degrees C. Most, but not all, revertants derived from these mutants had HA with the heat stability of wild-type virus, suggesting that the temperature sensitivity and the heat lability of HA were two pleiotropic manifestations of a single lesion in the HA gene.