We discuss a disorder that, although at first considered of possible fungal origin, now is known to be due to an alteration of erythrocytes induced by petrolatum-based antibiotic ointment. In this country, the structures of myospherulosis have been seen most often in tissues from the nose and paranasal sinuses following surgery in this region. Our studies show that both components of the vehicle of a commonly used tetracycline antibiotic ointment, lanolin and petrolatum, either separately or together, produce the structures of myospherulosis in vitro. It is recommended that a nonpetrolatum-based substance be used in nasal packing for purposes of hemostasis.