A phospholipid mixture was isolated from bovine seminal vesicle tissue. This phospholipid preparation contained the following prostaglandin precursor fatty acids: 4.7 percent 5,8,11-eicosatrienoic acid, 9.1 percent 5,8,11,14-eicosatetraenoic acid and 0.9 percent 5,8,11,14,17-eicosapentaenoic acid. With this phospholipid preparation as substrate phospholipase activity could be detected in different fractions of bovine seminal vesicle cells. The highest phospholipase activity was found in the microsomal fraction, less in the mitochondria and essentially no activity in the cytosol. With a microsome preparation (acetone powder) of bovine seminal vesicles the influence of certain effectors on the phospholipase activity was investigated. Prostaglandins E 2 and F 2a (10 -5 м) showed 19 and 41 percent inhibition, DB-c-AMP (2 × 10 -9 м) 54 percent, c-AMP (10 -9 м) 22 percent, epinephrine (10 -4 м) 60 percent, testosterone (10 -5 м) 25 percent inhibition of the phospholipase activity. Whereas ethanol exhibits also a strong inhibition on the phospholipase activity, it shows a pronounced activation of the prostaglandin synthetase.