Hygrophorus eburneus, commonly known as the ivory waxy cap or the cowboy's handkerchief, is a species of edible mushroom in the waxgill family of fungi.

The fruit bodies are medium-sized, pure white, and when wet are covered in a layer of slime thick enough to make the mushroom difficult to pick up.

The gills are broadly attached to the stem or running down it; as the family name suggests, they feel waxy when rubbed between the fingers.

Like all Hygrophorus species, the fungus is mycorrhizal—having a symbiotic association whereby the underground fungal mycelia penetrate and exchange nutrients with tree roots.

[4] The fungus has also been named Limacium eburneum by Paul Kummer in 1871,[5] when he raised the tribes of Fries to the rank of genus, and Gymnopus eburneus by Samuel Frederick Gray in 1821.

The gills are somewhat arcuate-decurrent, meaning they are shaped like a bow, curving upward and then running down the stem for a short distance.

The gills are moderately broad, broadest near the stem, narrowed in front, pure white, slightly yellowish or buff with age or when dried.

Observing with a light microscope reveals additional details: spores are ellipsoid, smooth, and measure 6–8 by 3.5–5 μm.

The cap cuticle is made of gelatinous, narrow (3–6 μm) hyphae which are repent (bent over) but typically with some erect free ends.

[12] A lookalike species of Hygrophorus eburneus is H. piceae, which differs by having a less slimy cap, dry to slightly viscid stem, and frequent association with spruce.

[23] The compound (E)-4-oxohexadec-2-enoic acid has been investigated for potential use as a fungicide against the oomycete species Phytophthora infestans, a causal agent of potato and tomato late blight disease.

[24] Additional secondary metabolites discovered in H. eburneus include the ceramide compound named hygrophamide ((2S,3R,4R,2'R)-2-(2'-hydroxy-9'Z-ene-tetracosanoylamino)-octadecane-1,3,4-triol),[25] and the β-carboline alkaloids known as harmane and norharmane.