A distinguishing microscopic characteristic is the near absence of large, spherical cells called sphaerocysts that are otherwise common in Lactarius species.

The species was first described by American mycologist Charles Horton Peck in 1907, based on specimens collected at Stanford University in California.

[4] The cap of L. rufulus is 3–10 cm (1.2–3.9 in) wide, broadly convex, becoming flattened and eventually shallowly funnel-shaped, sometimes with a slight umbo.

[nb 2] American author David Arora suggests that the fruit bodies may be used in a manner similar to the candy cap mushrooms, despite being not as fragrant.

[4] Lactarius rubidus is similar in appearance to L. rufulus, but it has watery to whey-like latex and develops a strong odor of maple syrup or butterscotch when dried.

The flesh and latex of Lactarius rufus have a strongly acrid taste, and its fruit bodies are stouter and lack an umbo.

[6] Additionally, L. rufus typically grows in a caespitose manner—with the fruit bodies clustered at a common base,[1] and has more pallid colors and a more intense odor than L. rufulus.

The fruit bodies of L. rufulus grow scattered or in groups on the ground under oak, usually from January to March; their appearance is uncommon.