A B S T R A C T A new human antigen is reported which is present only on blood neutrophils. A neutrophil-specific antigen, designated NAI, has previously been identified in two unrelated families, and was shown to be involved in fetomaternal incompatibility and the development of isoimmune neonatal neutropenia in five newborns. In the present paper, a second antigen, designated NB1, is identified in four families with seven affected children. Antibodies that react with this second antigen are shown to produce selective agglutination of neutrophils but not other blood cells. They are neither absorbed by cells prepared from solid tissues nor by non-neutrophilic blood cells.By family and population studies, NB is shown to be distinct from NA, representing an independent genetic locus. 68% of the New York population are homozygous for NB1, 29% heterozygous, and 3% negative. The NB locus is shown to be independent from those of HL-A and other known leukocyte antigens. No evidence for linkage between NA, NB, and red cell antigens was obtained.