U7 snRNP particles were bound in vitro to H4 pre-mRNA using immunoassay experiments. 2. The processing of in vitro H4 pre-mRNAs is an SmRNP-dependent reaction. 3. U7 RNA was isolated as a unique species from the complex H4-pre-mRNA-snRNPs, which indicates its involvement in the processing of histone pre-mRNAs.