Macrophage migration inhibition responses of mice immunized with mutant polyoma viruses or with cells transformed and/or infected by them have been studied. The macrophage migration inhibition reaction revealed individual differences. In several cases, mice immunized with a mutant virus responded preferentially or exclusively to extracts of cells transformed or infected with the corresponding mutant. Moreover, in the macrophage migration inhibition test, mutant viruses were usually less immunogenic than were the corresponding transformed-infected cells.