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The induction of nitric oxide in human monocytes during mycobacterial infection has been a controversial issue. This study describes a comparative evaluation of the colorimetric and fluorometric methods for the detection of NO in response toMycobacterium tuberculosis(MTB) infection in human peripheral blood-derived monocytes (PBM) and in U937, a human monocyte-derived cell line. MTB was grown in monocyte culturesin vitrofor 7 or 10 days. RPMI 1640 medium, without antibiotics and supplemented withl-arginine, Hepes, 5% human AB serum, and tetrahydrobiopterin was used to support monocyte growth. As early as 72 h after infection, soluble nitrite was detectable in the medium using the fluorometric assay with diaminonaphthalene (DAN). Early induction of NO correlated with an increase in the levels of iNOS mRNA as quantitated by RT–PCR. NO levels increased progressively up to day 10 (PBM) or day …