Three hundred and seventy-seven single sera from children in the United Kingdom, Hong Kong, New Zealand, Gambia, Guatemala, and Kuwait were tested for antibodies to fastidious fecal adenoviruses (species 40 and 41) by neutralisation test using a variant of Ad41 (Ad41a). All children were less than 12 years of age, and at least 41.1% of the total tested had antibodies. One hundred and fifty-four of these sera were collected during the 1960s from London children attending day nurseries, and 54.5% had antibodies to Ad40 or Ad41. The proportion of seropositive children from an isolated Gambian village community in 1979 was similar to the proportion of seropositives in London children bled in 1979-1980.