A series of self-paced reading time experiments was performed to assess how characteristics of noun phrases (NPs) contribute to the difference in processing difficulty between object- and subject-extracted relative clauses. Structural semantic characteristics of the NP in the embedded clause (definite vs. indefinite and definite vs. generic) did not influence the magnitude of the processing difficulty even though corpus analysis showed a strong association between these NP classes and type of relative clause. Richness of lexical semantic content in a descriptive NP also had no influence on processing difficulty. However, the difference in processing difficulty was significantly reduced when a quantified pronoun appeared as the NP in the embedded clause. Together with previous findings, these results support the conclusion that NPs with common nouns differ in representational similarity from NPs consisting of proper names and pronouns, and that similarity in the memory representation of NPs contributes to the difficulty of processing syntactically complex sentences.