Naive CD4⁺ T cell populations rapidly produce small amounts of IL‐4 in response to T cell receptor‐mediated stimulation and may undergo Th2 differentiation without exogenous IL‐4. Whether this is due to autocrine IL‐4‐stimulation or the production of IL‐4 by an infrequent naive cell has not been determined. Here we show that single CD4⁺ T cells from RAG2−/− T cells receptor transgenic mice primed with their cognate antigen give rise to IL‐4‐producing cells at a similar frequency whether primed with or without added IL‐4, but not if anti‐IL‐4 is added to the culture. Thus, each founder cell or one or more of its early daughters can produce sufficient IL‐4 to drive Th2 differentiation. This indicates that autocrine IL‐4 production by naive CD4 T cells can drive the appearance of Th2 cells.