Monoclonal antibody (MAB) BH2C6 recognizes a plasma membrane antigen, the BH2-Ag, specifically expressed by human neutrophils. While studies with peripheral blood and bone marrow from healthy adults clearly demonstrate the absence of BH2-Ag from other cellular components except neutrophils, they also indicate that the BH2-Ag is expressed more strongly by mature than immature neutrophils. The purpose of this study was to determine the expression of the BH2-Ag by peripheral blood neutrophils from premature newborns to adults. Seventy-two donors were studied in six age groups: newborns <36 weeks of gestational age; newborns >36 weeks of gestational age; 0.5-2 years; 4-8 years; 12-17 years; >30 years. Expression of the BH2-Ag by peripheral blood neutrophils was examined by cytofluorography using MAB BH2-C6 directly labeled with fluorescein isothiocyanate (FITC). Neutrophils were reacted in parallel with FITC-MAB directed against CD11b, the alpha-chain of the CD11b/CD18 antigen (CR3). BH2-Ag is expressed by 98.3-99.6% of the neutrophils in all groups, and is absent on other blood cells, including those of very premature newborns. Statistical comparisons with respect to the mean fluorescence intensity of the FITC-MAB BH2C6 bound did not support a significant difference in the expression of BH2-Ag in any age group. CD11b expression was also detected in every individual studied and its mean fluorescence intensity correlated significantly with that of BH2Ag (p <0.001). The uniform presence of BH2Ag in every individual including a very premature infant suggests that BH2-Ag is likely to be an essential component of neutrophil development in humans. A highly significant correlation between the mean fluorescence intensity obtained with MAB BH2C6 and MAB CD11b suggests a possible interactive role of the two antigens in neutrophil development and/or function.