Summary: Measles virus was grown in tissue cultures prepared from human amnion, from HeLa cells and from monkey kidney cells. The identity of the virus was established by the marked rise in neutralizing antibodies found in convalescent serum obtained from measles patients. Growth curves for the virus were prepared and it was found that the initial virus inoculum could be very low without sacrificing titer, provided the time of harvest coincided with maximum cytopathogenicity. Somewhat higher infectivity titers were obtained when infected cultures were maintained at 32°–33.5°C, rather than at 37°C. A plaque assay for measles virus, using a plasma overlay, was described. This method appeared to be more sensitive and more useful for quantitation than the roller tube assay. Using the plaque assay, a minimum adsorption time of 1 hr for measles virus on HeLa cells was obtained.