Summary: A method for identification and serologic classification of dengue viruses using a plaque reduction neutralization test and monkey immune serum is described. Results of typing 40 strains confirm the usefulness of this method for classification of low passage strains. Adaptation of the isolates to suckling mice or tissue culture was not required and strains within each of the four dengue serotypes were readily identified. Cross neutralization tests using monkey antiserum confirmed the results of the one-way neutralization test using antiserum to reference dengue strains and also indicated antigenic variation within serotypes 1, 2 and 4.