SUMMARY: In this work we present WhichCyp, a tool for prediction of which cytochromes P450 isoforms (among 1A2, 2C9, 2C19, 2D6 and 3A4) a given molecule is likely to inhibit. The models are built from experimental high-throughput data using support vector machines and molecular signatures. AVAILABILITY: The WhichCyp server is freely available for use on the web at https://drug.ku.dk/whichcyp, where the WhichCyp Java program and source code is also available for download. CONTACT: pry@sund.ku.dk SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.