Chromosome studies were done on peripheral blood preparations from patients with three different viral infections, i.e., measles, chickenpox and mumps. Blood samples from subjects who were inoculated with live measles vaccine were also subjected to the study. In patients with viral infection, no significant evidence for either numerical or morphological changes of chromosomes was obtained, except in a few cases. There was also no chromosome damage nor depressed response to phytohemagglutinin in leukocytes from subjects who received immunization with attenuated measles vaccine. The nature of the discrepancy between the present result and the results of hitherto reported studies in regard to chromosomal aberrations in viral infection was discussed.