Specific immunofluorescence was used to detect measles virus antigen in skin rashes. Cryostat sections of punch-biopsy specimens of the skin were stained with use of hyperimmune rabbit and horse antisera to measles virus, also conjugated with fluorescein isothiocyanate, served as controls. Measles virus was specifically demonstrated in 20 of 21 biopsy specimens taken within four days after the onset of exanthema. Measles virus antigen was also found in three of five biopsy specimens from nonexanthematous skin during the first four days of the exanthematous phase. The viral antigen was found in single cells or in clusters of cells in the surface epithelium, skin appendages, and corium. No viral antigen was detected in biopsy specimens taken five to six days after the onset of rash.