Erythrocytes, bone marrow-derived lymphocytes, monocytes, and granulocytes were shown to have a receptor activity for C4. Theis C4 receptor activity was studied in relation to the previously identified C3b and C3d receptors. By assay for inhibition of rosette formation by fluid-phase complement (C), only two different lymphocyte C receptors were demonstrated. The immune adherence receptor, the only one of the two shared in common with erythrocytes, was specific for C4 or the C3c region of C3b, but was unreactive with C3d. The other lymphocyte receptor, the C3d receptor, was specific for C3d fragments, but would also react to a lesser extent with the C3d region of uncleaved C3b. ThC3d receptor did not react with either C3c or C4. This specificity of the C3d receptor allowed certain cells which contained only C3d receptors to form rosettes with EAC1-3b and EAC1-3d, but not with EAC14. However, because C3d receptors bound EAC1-3d or C3d fragments more firmly than they did EAC1-3b or C3b fragments, many other types of cells containing only C3d receptors, formed rosettes with EAC1-3d but not with EAC1-3b. Erythrocytes and those lymphocytes which contained only immune adherence receptors, formed rosettes with EAC14 and EAC1-3D but not with EAC1-3d. A double-label assay was devised for the simultaneous detection of both types of C receptors on individual lymphocytes. This assay involved fluorescence labeling of one of the two C receptors with soluble C fragments in combination with the usual rosette method for labeling the other type of C receptor. With this double-label assay, it was observed that the two different lymphocyte C receptors capped independently and thus were located on different molecules which could each move through the fluid membrane matrix independently of the other.