The seroepidemiology of the defective adenovirus-associated satellite viruses was studied by use of a neutralization test that has been shown to be much more sensitive than the complement-fixation (CF) test. Less than 25% of the subjects in any age group were positive for any type of satellite virus CF antibody. The neutralization test revealed serological evidence of satellite infection in almost 80% of the subjects in some age groups. Studies of geometric mean antibody titers by neutralization showed that antibody against type 1 and the type 2-3 complex reached peak levels at 2 to 3 years of age; antibody against types 2 and 3 reached mean titers of 500 to 800, whereas mean antibody levels of only 200 were found for type 1. Type 4 antibodies were rarely noted in human sera. In a companion study in monkeys, monospecific responses to types 1 and 4 were found only in rhesus and green monkey sera, respectively. These data suggest that the type 2-3 complex may be the only naturally occurring satellite serotype which infects man and that antibodies noted against types 1 and 4 may be the result of cross-reactions. No satellite viruses were isolated from throat or rectal swabs of 100 children between 6 months and 3 years of age. However, the patterns of adenovirus and enterovirus isolations from these specimens were typical of previous studies in this laboratory.