In vitro activity: Omeprazole potently induces cytochrome P4501A1 mRNA expression in primary human hepatocytes, whereas this effect is not detected in mouse primary hepatocytes. Omeprazole induces transcription of reporter genes via the xenobiotic response element that is recognized by the ligand-activated dioxin receptor in human hepatoma cells. Omeprazole causes a strong inhibition of basal natural killer (NK) activity in spleen cells (SC) from untreated CD2F1 mice. Omeprazole causes a rapid, strong effect on various types of cytotoxic lymphocytes ranging from cytotoxicity inhibition to irreversible cell damage. Omeprazole induces a significant inhibition of cytotoxic activity of all types of effector cells after 30 min incubation. Omeprazole decreases the activation ofosteoclasts but increases that of osteoblasts in vitro, in part causing an osteopetrosis-like effect.