Five pairs of juvenile, dengue virus-susceptible rhesus monkeys were given normal or dengue-immune human cord-blood serum injected intravenously to a final dilution of 1:300. The pool of immune human cord-blood serum had a titer of antibody to dengue type 2 virus (D2V) of 1:140 in the plaque-reduction neutralization test and a titer of human monocyte infection enhancement of greater than 1:2,000,000. Fifteen minutes after inoculation of serum, animals were infected with D2V (strain no. 16681). Daily titers of viremia were always higher in the animals that had received antiserum to D2V than in animals that had received normal cord-blood serum. Ratios of infection enhancement ranged from 2.7 to 51.4. The demonstration of antibody dependence of dengue virus infection in subhuman primates--a complex, outbred experimental host--supports the hypothesis that the severity of dengue in humans is regulated by antibody.