Monocyte phagocytic activity was tested in 10 healthy term newborns using a latex phagocytic assay. The baseline phagocytic activity in the cord blood monocytes (37.7 +/- 3.8%) was comparable to adult controls (38.5 +/- 9.4%). When enhanced with 1,000 U/ml of alpha-interferon, the phagocytic activity was lower in cord blood monocytes (40.5 +/- 3.4%) than in the adult controls (47.9 +/- 10.6%), but this difference reached statistical significance only when the cord blood monocytes were enhanced with 2,000 U/ml reaching 43.6 +/- 4.3% in cord blood monocytes, as compared to 54.6 +/- 9.6% in adult controls. These findings may explain an inherent functional deficiency in the neonatal mononuclear phagocytic system which may not be evident during the quiescent phase of monocyte/macrophage activity, but may become apparent during an infectious challenge.