Growth was fastest in mice fed yogurt. There was no difference in the effects of live or heated yogurt on the ponderal curve or the organ weight of vaccinated or unvaccinated mice. Sera antitoxin levels were slightly elevated in animals fed live yogurt and given an antitetanus vaccination at the particular concentrations used. Significantly high IgG2a immunoglobulin levels were found only in unvaccinated mice fed live yogurt. Histological examination before, and particularly after, antitetanus vaccination confirmed these findings in the thymus and especially in the spleen.