Fc receptors are a family of membrane-associated and soluble glycoproteins that mediate a vast array of functions triggered by immune complexes. The structures of murine and human Fc gamma and Fc epsilon receptors have been elucidated and the motifs involved in the activities that they mediate characterized during the past year. B-cell activation and differentiation may be enhanced by different Fc receptor isoforms either through an increased presentation of antigen associated with IgG (Fc gamma RIIb2, Fc gamma RIII, Fc epsilon RII), or the induction of cytokine synthesis by mast cells (Fc epsilon RI, Fc gamma RIII) and natural killer cells (Fc gamma RIII). Conversely, the crosslinking of Fc gamma RIIb1 to membrane Ig inhibits B-cell activation. Soluble forms of Fc receptor also regulate antibody production by enhancing interleukin-4-induced IgE synthesis (Fc epsilon RII) or inhibiting IgG synthesis (Fc gamma R). Different structural motifs are responsible for the different biological activities of each Fc receptor isoform.