A mouse IgG1 monoclonal antibody (MAb) UMRh, was prepared by immunizing Balb/c mice with the Jurkat T cell acute lymphoblastic leukemia (T-ALL) cell line. The MAb UMRh is directed against a widely distributed Rh-related cell surface antigen, present on red blood cells (RBCs) expressing the more common Rh phenotypes. The antigen has reduced expression on RBCs of -D-, DCW-/DCW-, Rhmod and Rhnull phenotypes. UMR immunoblotted a unique pattern on RBC membrane preparations of two bands at 40 and 43 kD and a diffuse pattern extending upward to about 55 kD. The UMRh antigen is also present on peripheral blood mononuclear cells, granulocytes, platelets, leukemic cells of T cell, B cell and myeloid origins, hematopoietic stem cells, and two tumor lines (lung and colon carcinoma). The number of UMRh sites per RBC (CDe/ce) was determined to be 5,519 copies/cell, whereas the sites on a -D- phenotype RBC were 1,096 copies/cell. A T-ALL line (CEM) expressed 333,364 copies/cell and a myeloid line (KG-1) expressed 90,913 copies/cell. Several Rh-related murine MAbs have been described, but our data indicates that UMRh recognizes a previously uncharacterized Rh-related specificity.