Rpp20 and Rpp25 are subunits of the human RNase MRP and RNase P endoribonucleases belonging to the Alba superfamily of nucleic acid binding proteins. These proteins, which bind very strongly to each other, transiently associate with RNase MRP. Here, we show that the Rpp20-Rpp25 heterodimer is resistant to both high concentrations of salt and a nonionic detergent. The interaction of Rpp20 and Rpp25 with the P3 domain of the RNase MRP RNA appeared to be strongly enhanced by their heterodimerization. Coimmunoprecipitation experiments demonstrated that only a single copy of each of these proteins is associated with the RNase MRP and RNase P particles in HEp-2 cells. Both proteins accumulate in the nucleoli, which in case of Rpp20 is strongly dependent on its interaction with Rpp25. Finally, the results of overexpression and knock-down experiments indicate that their expression levels are codependent. Taken together, these data indicate that the Rpp20-Rpp25 heterodimerization regulates their RNA-binding activity, subcellular localization, and expression, which suggests that their interaction is also crucial for their role in RNase MRP/P function.