U5 small nuclear ribonucleoprotein (snRNP), purified from HeLa nuclear extracts (splicing extracts), shows a complex protein composition. In addition to the snRNP proteins B', B, D, D', E, F, and G, which are present in each of the major snRNPs U1, U2, U4/U6, and U5, U5 snRNP contains a number of unique proteins characterized by apparent molecular masses of 40, 52, 100, 102, 116, and 200 (mostly a double band) kDa. The latter set of proteins may be regarded as U5-specific for the following reasons. They are not only eluted specifically, together with snRNP particles, from anti-2,2,7-trimethylguanosine immunoaffinity columns by 7-methylguanosine, they also cofractionate with U5 snRNP during chromatography and, most importantly, in glycerol gradient centrifugation. These U5 snRNP particles show a high sedimentation constant of about 20S. U5 snRNPs that lack the U5-specific proteins are also found in nuclear extracts but have (in comparison) a lower sedimentation value of only 8-10S. Autoimmune sera from patients with systemic lupus erythematosus were identified that, on immunoblots with purified U5 snRNP proteins, reacted selectively with the 100- or 200-kDa proteins. This indicates that at least the high molecular mass U5-specific proteins are structurally distinct and not derived one from the other by proteolytic degradation. The existence of so many unique proteins in the U5 snRNP suggests that this snRNP particle may exert its function during splicing mainly by virtue of its protein components.