A newly modified enzyme focus forming assay (EFFA) has been developed for the titration of dengue viruses (DV). Infectious foci were stained with peroxidase labeled antibody and colorized with Karnovsky's solution. The foci were easy to count by the naked eye and were semipermanently preserved. The EFFA was compared either with conventional plaque assay or with suckling mouse intracerebral inoculation for the sensitivity and for the incubation period. The EFFA was higher in sensitivity and reduced the incubation period more than 2days in DV titration. Therefore the EFFA was useful for the titration especially of non/weakcytopathogenic viruses. The method was also useful for the ultrastructural study of viral replication in electron microscopy. The variables affecting the assay were investigated.