During the recently completed double-blind, placebo-controlled, randomized trial of recombinant interferon-gamma (rIFN-gamma) therapy in chronic granulomatous disease (CGD), a metabolic assay of neutrophil damage to Aspergillus fumigatus hyphae was used to monitor neutrophil function before and during therapy. In this assay, 5 x 10(4) conidia that had germinated into hyphae were exposed to 5 x 10(5), 15 x 10(5), or 50 x 10(5) CGD neutrophils. By analysis of variance, neutrophils from patients on rIFN-gamma were found to produce significantly more damage to hyphae than those from the placebo group (P less than .01). In subgroup analysis, this effect was best seen in the hyphae exposed to 50 x 10(5) CGD neutrophils, where neutrophils from patients receiving rIFN-gamma produced significantly more damage to the hyphae than those from the placebo group (P less than .05). In vivo rIFN-gamma therapy improves the ability of CGD neutrophils to damage Aspergillus fumigatus hyphae in an in vitro assay.