The most common test of acute toxicity is the LD 50 test. LD 50 means, if administered dose of drug to animal group, for experimental purpose for the e stimation of therapeutic effectiveness kills 50% of animals, than it means that particular dose of drug is lethal dose 50 (LD 50 ). It was developed in 1920’s and called “classical LD50” inv olved 100 animals for 5 dose-groups, later in 1981 it was modified by the Organization for Eco nomic Co-operation and Development (OECD) and reduced number upto 30 for 3 dose-groups . Methods to calculate LD50 values are - Litchfield and Wilcoxson, Reed-Muench, Miller-Taint er and Karber’s method. But all these methods require large number of animals. Factors wh ich affect the results of LD 50 are- Species, Age, Sex, Amount of food, Social environment etc. L D 50 study has some Limitations and results may vary greatly. Due to excess of animal sacrifice we should go to alternative methods which minimum number of animals is required. FRAME (Fund for the Replacement of Animals in Medical Experiment) believes that the lethal dose t est is unnecessarily cruel and scientifically invalid. Several countries, including the UK, have taken steps to ban the oral LD 50 . The OECD, the international governments’ advisory body abolis hed the requirement for the oral test in 2001. Three alternative methods and these are: Fixed Dose Procedure (FDP)-OECD TG 420, Acute Toxic Class method (ATC)-OECD TG 423, Up-and-Down P rocedure (UDP)-OECD TG 425. These methods only consider signs of toxicity in pl ace of death. Signs recorded during studies like; increased motor activity, anaesthesia, tremor s, arching and rolling. Alternative methods save numbers experimental animals.