IL-4 is a type I cytokine that is produced by activated T cells, mast cells, and basophils. It elicits many biological responses, two of which stand out as being of great importance: the regulation of helper T cell differentiation to the TH2 type and the regulation of IgE and IgGl production by В cells. Over the last 15 years, numerous studies using gene targeting and transgenic approaches have examined the structure of IL-4 and its in vivo role in infection and disease models. As a result of these studies, novel approaches either to inhibit IL-4 or to enhance its effects are being tested for their effects on disease models in animals and in human patient populations. Strategies to block IL-4 have shown promise in preventing or ameliorating allergic diseases. Strategies to target IL-4 to various tissues are being tested for effects on tumor growth and on skewing pathologic TH1 responses to nonpathologic TH2 responses. These strategies could ultimately lead to the ability to harness or eliminate the powerful effects of IL-4 for the improvement of human health.