A standardized extract mainly from rye pollen (Cernilton N) was tested in vitro on the inhibition of prostaglandin and leukotrien synthesis. The determination of the prostaglandin and leukotrien synthesis from labelled arachidonic acid was done in microsomes of ram seminal vesicles resp. in rat basophilic leukemia cells (RBL-1 cells). The water soluble resp. the fat soluble extract fraction from the whole pollen extract were tested separately. The radio-TLC separation of the reaction metabolites showed a dose dependent inhibition of the cyclo-oxygenase and the 5-lipoxygenase activity by the fat soluble pollen extract fraction. The IC50-values are 0.005 mg/ml resp. 0.08 mg/ml and similar to those of the also tested diclofenac. The water soluble fractions showed no effect in this test system. According to these in vitro results and the clinical experience so far with the pollen extract its therapeutic efficacy on benign prostate diseases is best explainable by the anticongestive resp. anti-inflammatory effect of the fat soluble fraction. Due to the different actions of prostaglandins and leucotrienes also relaxant and antiproliferative effects were conceivable.