Neutrophils are found in lesions of Nocardia asteroides infection, but neutrophils kill few nocardiae in vitro. For determination of neutrophil inhibition of nocardiae, neutrophils and nocardiae were incubated together. Filament formation and amino acid uptake by nocardiae were inhibited for 7.5 hr. Thereafter, nocardiae extended long filaments from within neutrophils to the outside and increased their rate of amino acid uptake. Addition of freshly isolated neutrophils at 7.5 hr of incubation prolonged the inhibition. Electron micrographs revealed that neutrophils phagocytosed nocardiae but that most nocardiae did not appear damaged. Formalin and 2-deoxy-D-glucose abrogated the inhibition by neutrophils. Lysozyme and granule-cationic proteins inhibited amino acid uptake under some conditions. Lactoferrin and lactic acid had no effect, and the oxidative metabolic burst was not required. These findings and clinical observations suggest that inhibition of N. asteroides by neutrophils may be important in vivo.