Measles viremia is thought to peak at onset of rash and diminish rapidly over the subsequent 2-3 days. The length of viremia and the proportion of peripheral blood mononuclear cells (PBMC) infected during measles were investigated in 8 adults. Blood was obtained from 7 patients between days 2 and 4 of rash. Five patients had repeat specimens obtained on day 6 or 7, and 1 patient had samples taken on days 6 and 10. Limiting dilutions of PBMC were cultivated with cord blood PBMC and stimulated with phytohemagglutinin. Virus was identified by syncytia formation and confirmed by immunofluorescent staining. Virus was isolated from all 8 patients. Four of 6 patients were still viremic at day 6 or 7 of rash. Titers ranged from 3 to 5623 TCID50/10(5) PBMC. Adults with measles may have prolonged viremia, and a large proportion of PBMC may be infected.