According to the US Food and Drugs Administration (Food and Drug Administration (2004) Challenge and opportunity on the critical path to new medical products.) "The inability to better assess and predict product safety leads to failures during clinical development and, occasionally, after marketing". This increases the cost of new drugs as clinical trials are even more expensive than pre-clinical testing.One relatively easy way of improving toxicity testing is to improve the design of animal experiments. A fundamental principle when designing an experiment is to control all variables except the one of interest: the treatment. Toxicologist and pharmacologists have widely ignored this principle by using genetically heterogeneous "outbred" rats and mice, increasing the chance of false-negative results. By using isogenic (inbred or F1 hybrid, see Note 1) rats and mice instead of outbred stocks the signal/noise ratio and the power of the experiments can be increased at little extra cost whilst using no more animals. Moreover, the power of the experiment can be further increased by using more than one strain, as this reduces the chance of selecting one which is resistant to the test chemical. This can also be done without increasing the total number of animals by using a factorial experimental design, e.g. if the ten outbred animals per treatment group in a 28-day toxicity test were replaced by two animals of each of five strains (still ten animals per treatment group) selected to be as genetically diverse as possible, this would increase the signal/noise ratio and power of the experiment. This would allow safety to be assessed using the most sensitive strain.Toxicologists should also consider making more use of the mouse instead of the rat. They are less costly to maintain, use less test substance, there are many inbred and genetically modified strains, and it is easier to identify gene loci controlling variation in response to xenobiotics in this species.We demonstrate here the advantage of using several inbred strains in two parallel studies of the haematological response to chloramphenicol at six dose levels with CD-1 outbred, or using four inbred strains of mice. Toxicity to the white blood cell lineage was easily detected using the inbred strains but not using the outbred stock, clearly showing the advantage of using the multi-inbred strain approach.