The plasma membranes of cod islet cells have been shown to contain both an ATPdependent and a GTP-dependent protein phosphokinase. The ATP-dependent enzyme is stimulated by cyclic AMP, glucose 6-phosphate, phosphoenolpyruvate and ouabain. The GTP-dependent enzyme was not affected by cyclic AMP, glucose 6-phosphate or phosphoenolpyruvate but was strongly inhibited by ATP. The plasma membrane of mouse /?-cells has been shown to contain adenylate cyclase and phosphodiesterase (Davis & Lazarus, 1972). The presence of these enzymes suggests that cyclic AMP may play a role in insulin secretion at the membrane. The most likely effector site for cyclic AMP is a protein phosphokinase. The purpose of the present study was to determine whether a protein phosphokinase enzyme was present in the membrane and also to determine the factors modulating its activity. Cod islets were used as a model for this study since large amounts of tissue were required. The islets were homogenized and fractionated into nuclear mitochondrial, heavy microsomal(5000-20000g), microsomal and soluble fractions. The microsomal fraction was then treated with 0.7% (w/v) sodium deoxycholate and centrifuged at 20000g. The supernatant was then dialysed against Tris-HC1 buffer, pH7.4, and used for the experiments. Enzyme markers showed that 71 %of the 5’-nucleotidase activity of the original hornogenate was present in the deoxycholate-solubilized fraction. This fraction also showed high adenylate cyclase and adenosine triphosphatase activity. Glucose 6-phosphatase activity was mostly found in the 500&20000g fraction and in the pellet remaining after deoxycholate solubilization; only 2% of the total homogenate activity was found in the prepared membrane fraction. The specific activity of the ATP-dependent protein phosphokinase was trebled after deoxycholate treatment of the rnicrosomal fraction. The assay was based on the rate of transfer of 32P from [Y~~P-]ATP or [Y-~~PIGTP to acid-precipitable protein (Kuo et at., 1970). The optimum temperature for the enzyme was 15°C and the pH optimum 6.5. Table 1 shows the effect of various substances on the incorporation of 32P into membrane proteins by the ATP-specific enzyme. Cyclic AMP was a potent stimulator of the enzyme. Its effect was abolished by the addition of a specific antibody for cyclic AMP. Glucose 6-phosphate and phosphoenolpyruvate were powerful stimulators of phosphorylation. Other hexose phosphates and pentose phosphates and intermediates of glycolysis were without effect. Ouabain,