The development of a live attenuated tetravalent dengue vaccine is currently the best strategy to obtain a vaccine against dengue viruses. The Mahidol University group developed candidate live attenuated vaccines by attenuation through serial passages in certified primary cell cultures. Dengue serotype 1, 2 and 4 viruses were developed in primary dog kidney cells, whereas dengue serotype 3 was serially passaged in primary African green monkey kidney cells. Tissue culture passaged strain viruses were subjected to biological marker studies. Candidate vaccines have been tested as monovalent (single virus), bivalent (two viruses), trivalent (three viruses) and tetravalent (all four serotype viruses) vaccines in Thai volunteers. They were found to be safe and immunogenic in both adults and children. The Mahidol live attenuated dengue 2 virus was also tested in American volunteers and resulted in good immune response indistinguishable from those induced in Thai volunteers. The master seeds from the four live attenuated virus strains developed were provided to Pasteur Merieux Connaught of France for production on an industrial scale following good manufacturing practice guidelines.