The yeast form of the dimorphic fungus Histoplasma capsulatum survives within macrophages after phagocytosis. To do so, it must avoid, inhibit, or resist a variety of toxic oxygen metabolites. Using ferricytochrome c reduction to assay superoxide release, we examined the response of mouse macrophages to the yeast form of various H. capsulatum strains. Doses of zymosan as low as 20 particles per macrophage elicited superoxide, whereas H. capsulatum failed to induce superoxide even at 160 yeast cells per macrophage. This phenomenon was observed with two virulent strains of H. capsulatum (G217B and G186A) and with an avirulent variant of G186A. Over a 15- to 150-min observation period, zymosan stimulated increasing reduction of ferricytochrome c, but H. capsulatum did not. When added concurrently with zymosan, H. capsulatum had no effect on superoxide production. Therefore, H. capsulatum was unable either to inactivate the oxygen radical or inhibit host cell superoxide response to other competent stimuli. Enzymatically generated superoxide reduced ferricytochrome c even in the presence of H. capsulatum, again implying that the organism does not readily inactivate superoxide. This experiment also demonstrated that the yeast did not interfere with the assay used. Thus, rather than inhibiting superoxide generation or inactivating the anion, H. capsulatum yeast cells appear to avoid the toxic effects of superoxide by failing to trigger its release.