Two new murine monoclonal antibodies were prepared by hybridoma technique after immunization with the immature pluripotent leukemia cell line K562. The monoclonal antibody Bra10G (IgG2b) reacted in a non-lineage pattern with all examined hematopoietic neoplastic cell lines and peripheral blood cells (granulocytes, lymphocytes, erythrocytes) of healthy donors, with the exception of monoblastoid cell line U-937 and B lymphoma cell line Daudi. This monoclonal antibody immunoprecipitated an 18-20 kDa cell surface protein expressed also on the cell surface of examined non-hematopoietic (malignant glioma, melanoma and breast carcinoma) cell lines. These properties and the efficient inhibition of Bra10G binding to the cell surface of K562 cells by the reference CD59 monoclonal antibody (MEM-43) indicated that Bra10G belongs to the CD59 cluster of monoclonal antibodies which identify the human protectin molecule. The monoclonal antibody Bra7G (IgM) reacted with a 95 kDa cell surface protein expressed on hematopoietic cells (with the exception of erythrocytes) and was absent on the examined non-hematopoietic neoplastic cell lines. These data together with a partial inhibition of Bra7G binding by the reference CD-43 monoclonal antibody suggested the CD43 (leukosialin, sialophorin) specificity of this monoclonal antibody.