Based on cDNA sequence data epsilon chain-specific antisense oligonucleotides were synthesized and checked on in vitro IgE production. Using peripheral blood cells from a hypereosinophilic patient and a human IgE myeloma cell line, U266, marked reduction of in vitro IgE production measured by PRIST was observed. The effect of epsilon antisenses proved to be isotype specific since IgG production by both peripheral blood cells and a lymphoma cell line, CESS, was not affected. Moreover, the expression of other markers on U266 (interleukin-6 receptor and gp130) were not influenced by epsilon-specific antisense oligonucleotides.