Fc receptors have been shown to be present in human placental tissue with properties distinct from those on macrophages and lymphocytes. A single class of receptor was observed with an intrinsic affinity of 4 × 106 M-1 for human IgG1. The order of affinity for IgG subclasses was IgG1 = IgG3 τ; IgG4 τ; IgG2. IgA and IgM were not bound. Fc from IgG1 bound with the same affinity as the whole molecule and to the same number of receptor sites, 2 × 1012/mg placental protein. IgG1 was no longer bound after mild reduction and alkylation whereas the binding of Fc was unaffected by this treatment. Neither Cγ2 nor Cγ3, the two domains which comprise the Fc region of IgG1, bound to the placental receptor. This implies that this Fc receptor is unlike those found on most cell surfaces and that placental binding is an exception to the theory that each domain has evolved to perform independent functions.