Basophils are one of effector cells in type 2 immune responses that critically are associated with allergic inflammation and infections with helminth parasites. Recent findings that basophils produce a large amount of Th2 cytokines (IL-4 and IL-6) has provided new insights into the possible role of basophils in the initiation phase, in addition to the effector phase, of type 2 immune responses. However, basophils occupy only 1% or less in blood, bone marrow and spleen, making it difficult to analyze their function. Here we establish a method for expressing exogenous proteins and their mutants in cultured basophils through retroviral infection. In combination with cultured basophils from gene-manipulated mice, this method, overcoming the paucity of basophils, allows us to examine the requirement of various signal transducers in interleukin 3-induced production of Th2 cytokines.