Three families in which cases of poliomyelitis had recently occurred were studied for the extent of subclinical infection and the presence of serum antibodies to both the mouse-adapted Lansing strain and the specific strain of poliomyelitis isolated from members of the family.

Virus was recovered from the stools of all 5 cases and from 8 of 17 familial associates at the time of the first collection of specimens. Only two of the associates were still carrying virus 3 weeks later.

The individuals from whom virus was recovered were younger than 11 years of age with two exceptions, one of them a frank case.

Antibodies to the mouse-adapted Lansing strain of poliomyelitis virus were demonstrated in the sera of every person 11 years of age or older but in none from individuals under this age with the possible exception of one whose second specimen was weakly positive.

Antibodies for the specific agent isolated from each family were likewise found at the beginning of the study in every person over 11 years of age but also in 4 of 9 under that age. The neutralization indices of sera of the second collection were generally higher than those of the sera collected soon after the disease occurred in the family, reflecting the antigenic stimulus of the "family" viruses.

The inverse relationship of demonstrable antibodies to virus isolations strongly suggests that the presence of serum antibodies is effective in limiting subclinical infection with poliomyelitis virus.

The question of cross-immunity in protection against infection with poliomyelitis virus is discussed.

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