Ascomycin, also called Immunomycin, FR-900520, FK520, is an ethyl analog of tacrolimus (FK506) with strong immunosuppressant properties.

Additionally, ascomycin preferentially inhibits the activation of mast cells, an important cellular component of the atopic response.

Ascomycin produces a more selective immunomodulatory effect in that it inhibits the elicitation phase of allergic contact dermatitis but does not impair the primary immune response when administered systemically.

[2] Ascomycin is produced by the fermentation of certain strains of Streptomyces hygroscopicus.

[3] Ascomycin is also the name of a fictional "antiagathic" (anti-aging) drug in James Blish's future history Cities in Flight.