Heterogeneous expression of three classes of Fc gamma receptor (Fc gamma RI, IIa, IIb, and III) in the human placenta and decidua was examined by Northern blot hybridization and cDNA amplification analysis by polymerase chain reaction. Messenger RNA of Fc gamma RI, IIa and III genes were consistently expressed in the human placenta in all trimesters of gestation. The transcripts of the Fc gamma RIIb gene, on the other hand, dramatically increased in placentae at the second and third trimesters. This characteristic expression of Fc gamma RIIb after 20 gestational weeks was confirmed by sequential cDNA amplification analysis. Fc gamma RI, IIa and III mRNAs, but not Fc gamma RIIb, were also detected in the human decidua. Interestingly, while Fc gamma R mRNA could be induced in uterine endometrium by pseudopregnancy therapy using estrogen and progesterone, there was no detectable mRNA in hormone-unprimed normal endometrium. These findings suggest that Fc gamma Rs expressed at the feto-maternal interface can be transcriptionally regulated by sex steroid hormones as multifunctional molecules. In addition, the Fc gamma RIIb molecule is predominantly produced by placental tissues after the mid-trimester of gestation and possibly plays an important role in the transport of IgG molecules from mother to fetus.