AbstractWe describe a population of nonadherent cells in neonatal cord blood that, upon in vitro cultivation, develop into monocyte‐macrophages. These cells initially are negative for nonspecific esterase cytoplasmic activity, lack the monocyte marker MO.2, fall into smaller, nonmonocytic cell size areas, as determined by fluorescence‐activated cell sorter (FACS)‐assisted size analysis, and differentiate into macrophages under nonstimulatory culture conditions (in the absence of exogenous colony stimulating factors,<0.1 ng/ml endotoxin, and growth in suspension). In contrast to the adherent, committed macrophage precursors in cord blood, which differentiate into macrophages after 2‐3 days of culture, the nonadherent precursor does not acquire monocyte‐macrophage characteristics until day 14 of culture. Earlier induction is achieved by adding the monocyte‐activating agents lipopolysaccharide or 1,25 dihydroxyvitamin D3 to cultures.