When the cloned h22 histone gene repeat unit of the sea urchin Psammechinus miliaris is injected into the frog oocyte nucleus, transcription initiates faithfully in the promoter region of the H3 gene, but transcription termination or RNA processing to yield genuine H3 mRNA 3′ ends is very inefficient. The generation of 3′ ends can be enhanced by the injection of a small sea urchin poly (A)- RNA into the oocyte cytoplasm one day prior to injection of the histone gene repeat unit. This RNA is ∼60 nucleotides in length and appears to be present in relatively low abundance in embryos of both Psammechinus and Paracentrotus. RNA extracted from a nuclear DEAE fraction, previously shown to complement the lesion in H3 gene expression, also causes the appearance of true 3′ ends of H3 mRNA. Furthermore, RNA extracted from our 12S "termination" factor includes an RNA species comigrating with the 60 nucleotide RNA from poly (A)- RNA. We suggest that the 12S nuclear "termination" component is in fact a small nuclear RNP.