The Fc gamma receptor (FcR) on normal human B lymphocytes was studied by using electron-microscopic monovalent ligands made of one ferritin (Fer) and one rabbit IgG anti-Fer. The binding of ligands was ascertained with the modified Coombs mixed antiglobulin reaction. The receptors were found clustered at 0 degree C, which was not affected even by examining the coated cells without washing away excess of the ligand solution, or by cross-linking bound ligands with a second antibody. The clusters varied in length, measuring up to 220 nm. Most of them were less than 110 nm long, and were seen as spots. When lymphocytes were preincubated with both cytochalasin B and colchicine, the clustering of FcR was not affected. Our findings show that FcR on B lymphocytes are naturally aggregated, which is not primarily mediated with the cytoskeletons made of actin microfilaments and microtubules.