Abstract
30 rhesus monkeys (Macaca mulatta), born into a breeding colony, received measles vaccination when aged between 3 and 26 months; 28 of them were re-vaccinated 5 to 7 months later. Measles virus antibody was measured by haemagglutination-inhibition at the time of each vaccination and again 3 to 6 weeks later. Only 2 out of 9 animals aged less than 6 months responded to vaccination, whereas 10 out of 14 older animals showed 4-fold or greater rises. Re-vaccination of the younger group when aged 8 to 10 months resulted in a marked rise in antibody.