Abstract 1. 1. Omeprazole, an antiulcer drug, inhibits the gastric acid pump via blocking the parietal cell (H + + K +)-ATPase. Omeprazole was also reported to have an inhibitory action on polymorphonuclear neutrophil activities. In the present study the potential effect of omeprazole on human natural killer cell (NK) activity was investigated. 2. 2. Omeprazole decreased NK cytotoxic activity in a dose-dependent manner. 3. 3. Degraded omeprazole showed a similar action. 4. 4. In vitro NK inhibitory action of omeprazole and its acid-degraded form was observed at the concentrations equal and higher than 18 μM (micromolar). 5. 5. NK inhibitory action of omeprazole was recovered to 75% by washing away of the agent. 6. 6. Omeprazole decreased the conjugate formation of effector and target cells by 50% at the concentration of 288 μM.