The superficial lesions, non-occupational character, and world-wide distribution and self-limited course of the diseases caused by parasitic fungi are contrasted with the deep-seated lesions, occupational character, restricted geographic distribution and progressive course of the diseases caused by saprophytic fungi. Injury to the tissues in both types of mycoses is attributed to allergic sensitization; the difference in the type of disease being due to differences in the seat of the allergic reaction. These allergic reactions are thought to be caused by antigen-like substances secreted by pathogenic fungi, which have specific properties not common to all antigens. These substances may possess specific toxophore groups, the allergic reactions they provoke being similar to toxin-hypersensitivity rather than to anaphylaxis or atopy.