An important asset of any therapy is the absence of secondary effects on host defenses. In the lung, bronchoalveolar macrophages (BAM) are a vital phagocytic defense against group B streptococci (GBS). GBS pneumonia is associated with pulmonary hypertension of the newborn (PHN), a condition potentially treatable with inhaled nitric oxide (NO). In vitro studies, however, show that NO donors can either enhance or hinder macrophage functions. Thus, we examined the effects of inhaled NO on anti-GBS activity by BAM. For 24 hours, 2-week-old piglets were exposed to either room air (RA),≥94% inspired oxygen (designated Hyperoxia), H + 5 parts per million (ppm) NO, or H + 50 ppm NO [n = 5 animals/group]. The exposure chamber had two beds of activated charcoal to trap nitrogen dioxide [concentration≤1.0 ppm] and prevent macrophage injury and lung inflammation. After 24 hours, BAM were recovered by lung lavage. Isolated BAM were used to study a) phagocytosis [fluorescent quenching method] and b) killing of non-opsonized and complement-coated GBS [2-hour tumbling assay]. The respiratory burst of lavaged BAM was also measured [ferricytochrome c reduction by superoxide anion]. Our findings indicated that a 24-hour exposure to H and inhaled NO did not impair macrophage-related phagocytosis and killing of complement-coated GBS or superoxide anion production following stimulation with complement-coated zymosan. Unexpectedly, non-opsonized GBS were killed by BAM recovered from piglets treated with H + 50 ppm NO, while native GBS proliferated when incubated with BAM lavaged from room air-exposed animals[P<0.05 by ANOVA]. Macrophages lavaged from piglets treated with H + 50 ppm NO spontaneously released 5 times more superoxide anion than did cells from room air-exposed animals [P<0.05], suggesting the former cells were activated. We conclude that during the first 24 hours of life when infants are most likely to receive inhaled NO for GBS pneumonia and PHN, the therapy may initiate a non-specific enhancement of functions used by bronchoalveolar macrophages against bacteria. This would be particularly helpful to infants lacking transplacental antibody against GBS wherein an augmentation of innate phagocytic behavior may be critical to host defense.