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Abstract. For estrogen-responsive B-1F cells, established from estrogen-responsive mouse Leydig cell tumor, it has been reported that the 5-lipoxygenase (5-LOX) metabolic pathway appears to be associated with cell growth. The addition of 5-LOX inhibitor 2-(12-hydroxydodeca-5,10-diyl)-3,5,6-trimethyl-1,4-benzoquinone (AA861) to the medium resulted in a dose-dependent increase in cell yield as described previously. When the growth of the palpable tumors was measured, AA861 had stimulated in vivo tumor growth in adult male mouse inoculated B-1F cells. The effects of AA861 and 17ß-estradiol (E2) on the contents of various arachidonic acid metabolites in B-1F cells and their conditioned medium were examined. Although AA861 and E2 decreased the contents of leukotrienes (LTs), the two did not significantly change those of prostaglandins, thromboxan, prostacyclin, 12-hydroxyeicosatetraenoic acid (HETE) and 15-HETE. In immunohistochemical study B-1F cells show positive staining for 5-LOX in the E2-depleted condition, while E2 decreased the expression of 5-LOX. The decrease of the intensities of 79-kDa 5-LOX protein and 403-bp RT-PCR product bands was observed. The growth of Morpholino-anti oligo delivered B-1F cells was higher than that of Standard control oligo delivered cells. The delivery of Morpholino-anti oligo into B-1F cells caused the decrease of contents of LTs and 5-HETE in the cells and medium, and the reduction of 5-LOX activity. When LTD4 was added in the culture medium, the increasing concentrations of LTD4 resulted in a significant inhibition of cell yields of E2-treated B-1F cells. Morphological changes such as nuclear condensation and fragmentation, and DNA ladder pattern were demonstrated in E2-stimulated B-1F cells treated with LTD4 as well as in control cells cultured in the basal medium. These results implicate that 5-LOX at least plays an important role in the growth of B-1F cells and LD4 induces the apoptosis of B-1F cells.