11 volunteers from the American group were revaccinated with a second dose (10 6.1 suckling mouse ic LD50) of 17D yellow fever vaccine about 14 months after the primary dose No viraemia was detected from the 4th to the 7th day after revaccination. 6 of these volunteers had viraemia on 1 or more occasions after the primary vaccination, and also possessed neutralizing antibodies to yellow fever at the time of revaccination. There was a rise of yellow fever antibody titre (average 4.7-fold) 21 days after the revaccination, but the mean titre was lower than that after the primary vaccination. Na neutralizing antibodies were demonstrated for dengue types 1, 2 and West Nile viruses. The findings suggested that pre-existing antibodies to YF possessed by the volunteers suppressed the occurrence of viraemia and modified the homologous antibody response after revaccination. Revaccination also had not broadened the heterologous antibody response. Chan Yow Cheong.