Zeta’s rabbit recombinant antibody recognizes CD20, a protein of 30-33kDa. CD20 is a non-Ig differentiation antigen of B-cells, and CD20’s expression is restricted to normal and neoplastic B-cells, absent from all other leukocytes and tissues. CD20 is expressed by pre-B-cells and persists during all stages of B-cell maturation but is lost upon terminal differentiation into plasma cells. Zeta’s antibody to CD20 can be used for immunophenotyping of leukemia and malignant cells, B lymphocyte detection in peripheral blood, and B cell localization in tissues. The CD20 antibody reacts with most B-cells present in peripheral blood and lymphoid tissues and their derived lymphomas. In lymphoid tissue, germinal center blasts and B-immunoblasts are particularly reactive. Zeta’s CD20 antibody is reliable for determing B-cell phenotype in known lymphoid tissues including rare CD20-positive T-cell lymphomas. The CD20 antibody reactivity has also been noted with Reed-Sternberg cells in cases of Hodgkin s disease, particularly of predominant lymphocyte type.