Three sera containing antibodies recognizing a previously undescribed antigen on granulocytes were found during testing of sera from multiparous donors. All of the antibody producers were in good health. None had a history of transfusion. Using the granulocyte agglutination assay the sera recognize a single antigen which is not associated with the neutrophil antigens NA1, NA2, NB1, NC1, ND1, NE1, 5a, 5b, 9a, nor common red blood cell or HLA antigens. The three sera did not react with autologous cells or with the cells of the other antibody producers. Granulocytes from one antibody producer did not absorb antibody activity from any of the three sera. The antigen was found in large quantities on granulocytes and monocytes, in smaller quantities on T lymphocytes, and not on B lymphocytes, red cells, and platelets. The sera reacted with 340 of 343 random donors (99.1%) and were negative with the same donor cells. Family studies showed autosomal dominant inheritance of the antigen. Five of 12 sibs in three families lacked this antigen (not statistically different from the expected ratio). The calculated gene frequency for the gene controlling the production of this antigen is 0.906. There appeared to be no association to the HLA, NA or Rh loci or to the X or Y chromosomes. None of the infants of these three women showed clinical signs of alloimmune neonatal neutropenia.