A temperature-sensitive mutant (ts 1/1) with a defect in the hemagglutinin (HA) gene, which was obtained by undiluted passage of fowl plague virus (FPV) at 33 degrees, is described. At 33 degrees proteolytic cleavage of the abnormal HA yielded an altered HA2 (XHA2) which migrated ahead of the NS1 protein and lacked the complex oligosaccharide side chain. At the nonpermissive temperature of 40 degrees, the migration of the HA of ts 1/1 from the rough endoplasmic reticulum (RER) via the Golgi apparatus to the cell surface was rate limiting for virus maturation. The HA was only slowly cleaved and migrated during polyacrylamide gel electrophoresis ahead of the HA of wild type FPV. Some revertants of ts 1/1 exhibited the same protein pattern as the mutant, others resembled wild type FPV, while one revertant gave rise to a mixture of HA2 and XHA2 at 40 degrees. These results suggest that (1) the loss of the complex oligosaccharide side chain is not responsible for the ts phenotype, (2) the mutation is presumably not at the site where the oligosaccharide side chain is linked to the protein backbone, and (3) ts 1/1 presumably carries a mutation located in RNA segment 4, which by pseudoreversion (suppressor mutation) in the same gene leads to different ts+ phenotypes.