Though its colour and size vary, it is distinctive because it is slimy when wet and exudes copious amounts of milk.

Blennins, some of which have shown potential medical application, are derived from lactarane, a chemical so named because of their association with Lactarius.

[3] The specific epithet is derived from a Latin adjective blennius, meaning "slimy".

[7] Lange also described Lactarius blennius f. virescens, which is now recognised as nomen invalidum (an invalid name).

However today L. fluens is normally regarded as a related but separate species, distinguished by a pale cap margin, less sliminess and a deeper cream gill colour.

[11] Lactarius blennius has a flattened convex cap that is 4–10 cm (1.6–4 in) across that later becomes depressed in the centre.

[4] L. blennius spores leave a creamy print, and are elliptic with low warts joined by ridges with a small number of cross-connections, measuring from 6–9 by 5.5–7 μm.

[14] It forms an ectomycorrhizal association with trees,[14] and can grow on a wide variety of subsoil types,[15] but is more typical of acidic soil.

[20] Mycologist Roger Phillips claims that L. blennius is edible when cooked, but not recommended,[5] while others describe it as inedible[2][13] or even poisonous.