We investigated putative mechanisms by which human surfactant protein A (SP-A) effects killing of Klebsiella pneumoniae by human alveolar macrophages (AMs) isolated from bronchoalveolar lavagates of patients with transplanted lungs. Coincubation of AMs with human SP-A (25 μg/ml) and Klebsiella resulted in a 68% decrease in total colony forming units by 120 min compared with AMs infected with Klebsiella in the absence of SP-A, and this SP-A-mediated effect was abolished by preincubation with NG-monomethyl-L-arginine. Incubation of transplant AMs with SP-A increased intracellular Ca2+ concentration ([Ca2+]i) by 70% and nitrite and nitrate (NOx) production by 45% (from 0.24 ± 0.02 to 1.3 = 0.21 nmol·106 AMs-1·h-1). Preincubation with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid-acetoxy-methyl ester inhibited the increase in [Ca2+]i and abrogated the SP-A-mediated Klebsiella phagocytosis and killing. In contrast, incubation of AMs from normal volunteers with SP-A decreased both [Ca2+]i and NOx production and did not result in killing of Klebsiella. Significant killing of Klebsiella was also seen in a cell-free system by sustained production of peroxynitrite (>1 μM/min) at pH 5 but not at pH 7.4. These findings indicate that SP-A mediates pathogen killing by AMs from transplant lungs by stimulating phagocytosis and production of reactive oxygen-nitrogen intermediates.