A new method of detecting the C3b receptor is reported. A particular merit of this method is that anti-RBC rabbit antiserum is not required. Rosettes were formed with human B lymphocytes, B lymphoblasts and granulocytes, using sheep erythrocytes (SRBC) sensitized with fresh human serum (FHS). T lymphocytes and T lymphoblasts did not form rosettes. The percentage of cells forming rosettes with this method approximated the percentage of rosettes formed with EACm. However, FHS coated SRBC did not react with most cells of B cell type chronic lymphocytic leukemia (CLL), whereas EACm rosette formations showed a definite reaction. On the other hand, 34--58% of cells of chronic myelocytic leukemia (CML) bound with the indicator red cells. SRBC sensitized with fresh rabbit or guinea pig serum formed rosettes with PBL, tonsil cells, B lymphoblasts and granulocytes. Complement and IgM antibody were required for this reaction, as in EAC rosette formation.