The receptor for IgA, Fc alpha R, consists of one IgA-binding alpha chain and a signal-transducing dimeric FcR gamma chain. Immunoprecipitation with an anti-Fc alpha R alpha chain monoclonal antibody from the lysates of U937 cells (human monocytic cell line) revealed an association of 20 kd (unreduced) and 10 kd (reduced) molecules to Fc alpha R alpha chain on sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). These molecules were confirmed to be FcR gamma chain by immunoblotting probed with anti-FcR gamma chain antibody. A serial immunoprecipitation with both antibodies further ascertained the FcR gamma association with Fc alpha R alpha. The lysates precleared with anti-FcR gamma antibody were subjected to the second immunoprecipitation with an anti-Fc alpha R alpha monoclonal antibody. By this preclearance, FcR gamma disappeared, and the Fc alpha R alpha appeared to be significantly decreased on SDS-PAGE, suggesting that a part of Fc alpha R alpha was co-absorbed with FcR gamma. Therefore, it may be likely that Fc alpha R is expressed in two forms, namely, with or without FcR gamma. We next reconstituted the Fc alpha R alpha-FcR gamma association by introducing both chains into host cells. The expression of Fc alpha R alpha was achieved by introducing Fc alpha R alpha alone, and the cointroduction of FcR gamma did not enhance Fc alpha R alpha expression on the cell surface, suggesting again the occurrence of the two forms of FC alpha R.