In vitro assays for mutagenicity are an important feature of pre-clinical testing and form part of the current regulatory testing conducted early in drug development. They can also play a part in compound selection since mutagenic compounds can be eliminated from a range of potential candidates. Bacterial tests are particularly useful in this area because they generate results quickly, though their use may be limited because they can require up to 4 g of material. A scaled-down version of the Ames test has been developed which requires only approximately 20 mg of material. Initial experiences with this assay using a range of known mutagens and novel compounds have shown that the Miniscreen has similar sensitivity to the Ames test. The major exception is for those mutagens preferentially detected with strains TA1537 and TA1535, which, because of their low spontaneous counts, are not employed in the Miniscreen.