Closed pileus up to c. 16 x 12 mm, red-brown to purple-brown, in young specimens often very dark (Mu. 5 YR 2.5/2, K. & W. 9F4) at centre, paler towards margin (Mu. 7.5 YR 4/2, K. & W. 7D/E4), up to c. 30 mm in diam. when expanded. Lamellae narrowly adnate. Stipe 40-80 x 1-3 mm, white to greyish-white, pubescent from setulae.
Spores [140,7,7] 10.4-13.8 x 5.8-7.4 µm, av. L= 11.7-12.6, av. B= 6.3-7.0 µm, Q= 1.70-1.90, av. Q= 1.75-1.85, ellipsoid to ovoid; germ pore eccentric, c. 1.8 µm wide. Basidia 18-38 x 8-9 µm, 4-spored. Pseudoparaphyses (3-) 4-6 per basidium. Cheilocystidia globose, up to c. 40 µm wide. Pleurocystidia absent. Pileocystidia 60-150 x 10-18 µm, lageniform with cylindrical neck and clavate to (sub)capitate 7-12 µm wide apex. Sclerocystidia absent. Clamp-connections present.
In young and fresh stages C. plagioporus has a dark purple pileus. Microscopically the combination of subcapitate to capitate pileo- and caulocystidia and globose cheilocystidia is usually characteristic for this species. But unfortunately the tips of the pileocystidia and caulocystidia are sometimes only slightly enlarged and in these cases confusion with C. subpurpureus and C. fallax is possible (see also the discussions under these two species).
From a morphological point of view it is tempting to consider C. plagioporus, C. subpurpureus and C. fallax as variants of one and the same species. M. Lange (1952: 44), however, found C. plagioporus and C. subpurpureus completely intersterile. Some years later M. Lange & A.H. Smith (1953: 765) described the probably closely related C. fallax as a new species, mainly on account of its aberrant spore print colour ('reddish'), but without testing it against C. plagioporus and C. subpurpureus.
M. Lange & A.H. Smith (same publication p. 766-768) stated that because of the moderately eccentric germ pore and the dark spores measuring 11.75 x 6.05 śm on the average, C. ephemerus f. saturatus J. Lange (1939: 117) is identical with C. plagioporus. But many species of section Setulosi have such spores and J. Lange's forma may belong to any of them. J. Lange's coloured illustration (1939: pl. 160 D) does not help either, as macroscopically many species of subsect. Setulosi look very similar and this illustration could refer to several of them.