Immunoglobulin class switching is controlled by cytokines. Thus, interleukin‐4 (IL‐4) directs class switching to both IgG₁ and IgE. Consistent with this are the results reported here on restriction endonuclease analysis of active and inactive alleles of the IgH locus in IgE‐producing cells. In cells that were stimulated in vitro by lipopolysaccharide and IL‐4 the silent alleles preferentially switched to γ₁, whereas in cells that were stimulated by antigen in vivo both active and inactive alleles switched to γ1. Thirty percent of the recombined switch regions (S_μ/S_ϵ) contain S_γ1 sequences, which we interpret as footprints of a previous switch to γ₁. Since this percentage is a minimum estimate, between 30% and 100% of switching to ϵ must occur sequentially via γ₁.