The influence of larch (Larix decidua L.) sawdust extracts on arachidonate metabolism has been evaluated in vitro. Extracts of different polarities were tested for their ability to inhibit prostaglandin formation by COX-1 and COX-2 and LTB(4) formation by 5-LOX. The lipophilic n-heptane extract showed the highest activity (IC(50) values: COX-1, 5 microg/mL; COX-2, 0.1 microg/mL; LTB(4) assay, 11.1 microg/mL). A series of abietane, pimarane, and labdane type diterpenes isolated from this extract turned out to be potent inhibitors of LTB(4) product formation, whereas their COX inhibitory activity was less pronounced. From the abietane type diterpenes, compounds possessing a 7,13-abietadiene skeleton were better inhibitors of LTB(4) formation than those with an 8,11,13-abietatriene skeleton. Compounds bearing an OH group in position 4 were more active than those substituted with a COOH group, and methylation of the COOH group led to an almost complete loss of LTB(4) formation inhibitory activity.