FROM THE first reports1,2 of data obtained in careful clinical investigations of susceptible children to whom live HPV-77 African green monkey kidney (GMK) rubella vaccine was administered, several characteristics of the attenuated infection became apparent. As other investigators confirmed and extended, these initial results over a three-year period, the original features have been strikingly consistent.3-5 They include the successful establishment of an infection which is usually occult, the intermittent shedding of vaccine virus from the pharynx during the period from one to three weeks after immunization, the absence of communicability to susceptible contacts, a failure to develop detectable viremia, and the appearance in two to four weeks postinoculation of type-specific serum antibody with both hemagglutination-inhibiting (HI) and virus-neutralizing (VN) activities but no measurable complement-fixing antibody. In collaborating with Drs. Parkman and Meyer to enlarge the experience with this experimental vaccine, our continuing interest in the patterns of interaction