Several parameters have been established for the control of yellow fever vaccine produced in embryonated eggs at Fundaçāo Oswaldo Cruz, Brasil. A Vero cell plaque titration technique was used throughout this study. A local reference vaccine with an infectivity titre of 5.78 log10 ml-1(standard deviation 0.28) was routinely included in all tests. It was found that, while one cycle of freezing and thawing did not affect the virus titre, a second thawing reduced it. The titres of the embryo pulp and the embryo juice, virus losses during lyophilization cycles, the titre of the final product and the thermodegradation of the virus were measured and these parameters are being currently applied to every step of the production procedures with a view to strict quality control of yellow fever vaccine.