It was discovered that dengue 1 antibodies were able to neutralize virtually all American genotype dengue 2 viruses to high titer. Geometric mean titers observed in 34 dengue 1 sera when tested with and American genotype dengue 2 virus was 1:262 compared with 1:875 versus homologous dengue 1 virus. However, two strains of Asia genotype dengue 2 (one isolated in the Americas and the other in Asia) were poorly neutralized by antibodies to dengue 1.