For initial screening for antibodies against infl uenza virus, serum samples were treated with receptor-destroying enzyme as described (9) and tested by using a multispecies Infl uenza A Virus NP Antibody Inhibition Test (Virusys Corporation, Taneytown, MD, USA) according to manufacturer's instructions. ELISA results indicated nucleocapsid protein antibodies against infl uenza in samples from macaques from Cambodia (29.2%), Singapore (16.7%), Sulawesi (16.1%), Bangladesh (13.3%), and Java (6.0%) ( Table 1) . Antibodies were detected in animals 1-10 years of age at the time of sampling. No infl uenza virusspecifi c antibodies were detected from the 73 total samples from Gibraltar, perhaps because persons with infl uenza virus infection infrequently travel to the Upper Rock Reserve (healthy-visitor effect) (10) or perhaps because monkeys from Gibraltar are less susceptible to infection. Seroprevalence of antibodies against infl uenza A virus, by site and collection year, human and NHP population densities, and prevalence of avian infl uenza viruses are shown in Figure 2 .