We have previously characterized B1-Alu gene expression by microinjected Xenopus laevis oocytes. The transcription, endonucleolytic processing and its kinetics, nuclear transport kinetics, and subsequent cellular compartmentalization have been described previously (Adeniyi-Jones and Zasloff, Nature 317:81-84, 1985). Briefly, a B1-Alu gene is transcribed by RNA polymerase III to a 210-nucleotide (210nt) primary transcript which is processed to yield 135nt and 75nt RNAs. After processing, the 135nt RNA enters the cytoplasmic compartment, where it remains stable, while the 75nt RNA is degraded. In this report we characterize this pathway further and show that the RNAs involved are complexed with specific X. laevis proteins. The primary transcript was associated with an X. laevis protein of 63 kilodaltons (p63) as well as La, a protein known to be associated with RNA polymerase III transcripts. After processing, the cytoplasmic 135nt RNA remained associated only with the X. laevis p63 in the form of a small ribonucleoprotein. Human autoimmune antibodies were purified by affinity chromatography to X. laevis p63 and used to immunoprecipitate human ribonucleoprotein containing a 63-kilodalton polypeptide and small RNAs. These data suggest that Alu-analogous ribonucleoproteins and their metabolic pathways are conserved across species and provide insight as to their possible functions.