Structural homology between C3 and C5 as well as the C5 dependent enhancement of phagocytosis by human PMNL (Segerling, M and W. Opferkuch, Fed. Proc. 35, 273, 1976) prompted us to investigate the presence of binding sites for C5 on lymphoid cells carrying C3 receptors. Raji cells were mixed with solutions of varying concentrations of C5 (2.5–0.01 mg/ml), incubated for 20 min at 37°C and washed three times in buffer. These cells (Raji-C5) were then tested for rosette formation with EAC14oxy23b and EAC14oxy23d. Depending on the increasing concentration of C5 during the preincubation, Raji cells formed less rosettes: interaction with EAC14oxy23d could be depressed nearly completely by C5 (from 70% to 10%), while the interaction with EAC14oxy23b could never be reduced below a certain percentage (from 70% to 35%), This was in contrast to the action of C3, which could prevent Raji cells from reacting at all with either type of intermediate.