Sera from 155 children in a new housing project near Jerusalem were examined for antibodies against the three types of poliomyelitis. From the age of two to three years onwards more than 50 per cent of the sera showed antibodies against all three types of poliomyelitis. In the age group 6–7 years 95 per cent of the sera contained antibodies against all three types. Antibodies were more frequent in children of families where crowding and a low degree of cleanliness were present. Under the conditions observed no influence of the economical status of the family was apparent.