Methods for obtaining beneficial stable levels of circulating histamine are disclosed for use in methods for enhancing natural killer cell cytotoxicity. In such methods, a beneficial level of circulating histamine is attained and an agent whose ability to enhance natural killer cell cytotoxicity is augmented by histamine is administered. Alternatively, stable beneficial levels of circulating histamine can be attained in subjects receiving chemotherapy or antiviral treatment. The invention may also be employed in treatments combining histamine, agents which enhance natural killer cell cytotoxicity, and chemotherapeutic agents. Optimization of the delivery of histamine and substances which induce the release of endogenous histamine are also disclosed.