Histopathologic examination of liver from patients with yellow fever is often not diagnostic. We therefore compared 2 virus-specific assays applicable to fixed liver, in situ nucleic acid hybridization and an immunocytochemical [alkaline phosphatase-antialkaline phosphatase (APAAP)] technique. Yellow fever structural gene sequences were detected by use of 35S-labeled negative-sense RNA probe (but not by immunocytochemistry) in 11 of 17 livers from children with fatal illness during the 1965 epidemic in Senegal. These fixed liver samples had been stored at ambient temperatures for 23 years. Both techniques were diagnostic on tissues collected 15–37 months before testing. Immunocytochemistry is a practical procedure for rapid specific diagnosis of liver stored for months, whereas RNA-RNA hybridization is a sensitive technique which can be applied to material stored for years.