Since introduction of the pertussis vaccine in 1940's the morbidity and mortality due to the infection has been markedly reduced all over the world. However the adverse effects of the inactivated whole cell pertussis vaccine like pain, swelling at the site of injection, fever, vomiting anorexia, persistent crying & drowsiness have been the cause of great concern, till date. Also the safety concerns over the use of thiomersal as an inactivating agent as well preservative have been raised in the recent past. Studies in many countries have been initiated to reduce or replace thiomersal & using other inactivating agents in the vaccines. Limited studies have been conducted in India. The present study has been undertaken as an attempt to reduce the quantity of thiomersal and modification in the procedure of production of the pertussis vaccine to reduce the toxicity, to produce better quality of whole cell pertussis vaccine. To achieve this, at the time of production of the whole cell pertussis vaccine, at Kasauli, as per the standard procedure recommended by WHO, three parallel batches of the pertussis culture were inactivated in fermenter before harvesting and thiomersal was used only one time for suspending the bacterial mass after harvesting. The resultant modified vaccine so prepared when tested showed that it was of better quality as compared to the one produced by standard procedure, when tested in mice.