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Type III receptors for the Fc portion of IgG (Fc gamma RIII), initially characterized on macrophages and NK cells, are also expressed on several pre‐B cell lines. Surface expression of Fc gamma RIII requires the association of the ligand binding alpha‐chain with homodimeric gamma‐chains. Type II Fc gamma R is homologous to Fc gamma RIII alpha‐chain in the extracellular portion and differs in the transmembrane and cytoplasmic domains. The role of Fc gamma R in cell activation was investigated by expressing Fc gamma RIII and the lymphocyte‐specific b1 isoform of Fc gamma RII (Fc gamma RIIb1) in an Fc gamma R‐negative, sIgG‐positive B‐cell line. We found that, in contrast to Fc gamma RIIb1, Fc gamma RIII triggers the same events of cell activation as sIG i.e. Ca2+ mobilization, tyrosine phosphorylation and IL‐2 secretion. By expressing cytoplasmic domain‐lacking Fc gamma RIII alpha‐chain in the absence or in the presence of gamma‐chains, we demonstrated that cell activation via Fc gamma RIII requires the co‐expression of gamma‐chains, and is independent of the cytoplasmic portion of the alpha‐chain. Furthermore, the cytoplasmic portion of the gamma‐chain, fused to the extracellular and transmembrane domains of Fc gamma RII confers on the chimeric receptor the ability to trigger cell activation. Mutation of one tyrosine residue in the cytoplasmic domain of the gamma‐chain prevented triggering of cytoplasmic signals. We therefore demonstrate that a tyrosine‐containing motif, present in the cytoplasmic domain of the associated gamma‐chain, is necessary and sufficient to trigger cell activation via Fc gamma RIII.