The frequency of Fc-receptor positive B lymphocytes in human blood was investigated. Under the conditions used heat-aggregated gammaglobulin binding and EA(ox)-rosette formation labelled the same lymphocyte populations. Using various techniques, double marking and cell separations the proportion of Fc-receptor positive cells within the surface Ig carrying population was estimated to be between 11-8 and 36-2%. The proportion of SIg carrying cells within the population forming EA-rosettes was between 11 and 26-4%. This represents extreme values due to known technical circumstances.