During a measles epidemic in Panama, 453 persons with no previous history of measles were inoculated with a modified live-virus measles vaccine, and 356 others served as controls. About 70 to 80% of children under 5 years of age had some degree of fever, and about 50% of these developed a slight macular rash. During the following four and one-half months, three of the vaccinated group developed measles, compared with 32 among the controls. In addition, 388 persons were vaccinated with canine distemper vaccine, with 414 controls. None showed reactions to the vaccine. In the following three to four months, 14 cases of measles occurred in the vaccinated group and 25 among the controls. It is concluded that the measles vaccine was effective but caused some reaction, while the ability of this strain of distemper virus to protect against measles appears to be low.