Aspergillus fumigatus mycelium in untreated mice (N-mice) and cortisone acetate-treated mice (C-mice) has been quantified by chemical assay of fungal chitin. Cortisone pretreatment rendered mice more susceptible to infection by A. fumigatus (mean lethal dose at 20 days, congruent to 10(6) for N-mice; less than 10(4) for C-mice). In both N- and C-mice there was renal localization of mycelial infection at conidial doses less than the mean lethal dose. At a conidial dose greater than the mean lethal dose, mycelial infection was found in the kidneys and brain of N-mice and in the kidneys, liver, and heart of C-mice. Chitin assay results showed that A. fumigatus mycelium grew more rapidly in C-mice. It is suggested that the resistance of N-mice to mycelial development may be an important mechanism whereby natural resistance to A. fumigatus is conferred.