Adult BALB/c mice of both sexes were infected intranasally with 10(6) viable P. brasiliensis conidia. Animals were sacrificed at intervals up to 6 months and studied by histopathology and organ cultures. At the time of challenge lung sections showed that instilled conidia had reached the alveoli; at 12 h such conidia were transforming into yeast cells, with multiple buds appearing by 18 h. Initially, the cellular infiltrate was composed of polymorphonuclear leukocytes; 6 days later, lymphocytes, plasmocytes and macrophages predominated. Multinucleated giant cells appeared only after 6 weeks. The rate of pulmonary infection as determined by organ culture was high (82 of the 83 mice studied). The experimental infection was progressive as indicated by increasing numbers of viable fungi with time. The results of this study demonstrate that the conidia produced by P. brasiliensis mycelial form are infectious, producing active disease in healthy animals.