In culture, cytochrome P-450 levels fall rapidly with the result that hepatocytes are either used quickly or maintained in modified systems which prejudice their subsequent behaviour. In this study the effect of hypothermic preservation of hepatocytes on gelatin gels on levels of cytochrome P-450 was investigated. In marked contrast to conventional cultures, hypothermic preservation (10°) maintained, over a 6-day period, cytochrome P-450 at levels similar to those of the more stable cytochrome b5. Cell storage on gelatin at 25° was associated with a conversion of cytochrome P-450 to cytochrome P-420. The procedure at 10° provides a valuable tool for toxicity testing, hepatocyte conservation and distribution.