Expression of Fc receptors for IgE (FcER) or IgA (FcAR) on purified natural killer (NK) cells was investigated. No FcER+ and a few FcAR+ NK cells were detectable on freshly separated NK (NKH-1+) cells from normal donors. Incubation of NK cells with IgE-anti-IgE immune complexes or IgA-anti-IgA immune complexes induced up to 10 and 20% FcER+ or FcAR+ cells, respectively. These FcR were induced on CD3- but not on CD3+ NKH-1+ cells. In contrast, NK cells from patients with various dysgammaglobulinemias could not be induced to express FcER or FcAR corresponding to their abnormal circulating IgE and/or IgA levels. Enriched FcER+ or FcAR+ induced NK cell supernatants from normals enhanced IgE or IgA synthesis from Ig secreting B cell lines in an isotype-specific fashion without increasing proliferation. Thus NK cells, after interaction with specific Ig isotypes in complexes, express FcR and produce differentiation factors for that isotype.