Passive neutralizing antibodies specific to Coxsackie B-1 virus are transmitted from immune mice to the litters both before and after birth.Before birth the antibodies are transmitted via the mother"s blood; their presence could not be detected by the age of 17 days. After birth, the antibodies apparently are transmitted by milk. The ND50of these antibodies is maintained if the litter is suckled by an immune mother.Although the immune 4-day-old litters survive after intracerebral inoculation and there are no signs of paralysis or other severe damage to the CNS, the brain lesions on microscopical examination appear to be as extensive as in the non-immune controls. The brain tissues of the immune mice are spared if the virus is introduced by the subcutaneous route.The damage observed in the interscapular brown fat in either the immune 4- or the 17-day-old mice, irrespective of the route of virus inoculation, is considerably reduced or minimal, if the presence of the passive antibody can be demonstrated in the blood.When the offspring are born of and suckled by an immune mother, the multiplication of the Coxsackie B-1 virus in the brown fat appears to be completely blocked, and there is no evidence of an immediate antigenic stimulus by the challenge virus.