Herbals and other natural remedies could affect the disposition of conventional pharmaceuticals through inhibition of human cytochrome P-450 (CYP) enzymes. We have developed an approach to the problem of determining a critical potential for CYP enzyme inhibition by commercial herbal remedies etcetera using standardised extraction procedures in combination with commercially available human CYP enzyme 3A4, 2D6 and 2C19 inhibition assays. We present a survey of commercially available natural remedies on the local market using this approach together with a discussion on how to decide when further in vivo interaction studies may be warranted. We confirmed earlier findings on Hypericum (St. John's wort) and Echinacea purpurea activities, and report that extracts of Valeriana as well as a fish oil preparation were potent inhibitors of all tested enzymes. As a first estimate to assess the relevance of in vitro findings, we have chosen to express the inhibitory potency as the volume that the extractable inhibitory activity for a recommended human dose could be diluted into to yield a half-maximal inhibition--litres per dose unit. We propose that preparations for which this measure approaches four litres/dose unit, i.e. corresponding to the human blood volume, should be investigated further for potential enzyme interaction with pharmaceuticals.