Human nuclear cyclophilin 33 (hCyP33) was the first protein which was found to contain an RNA-binding motif and a PPIase domain. It was not known what cellular and physiological roles are played by the RNA-binding activity as well as the PPIase activity of hCyP33. In this paper, we investigated the binding specificity of hCyP33 to different cellular RNA using ion-exchange chromatography and affinity adsorption. Furthermore, the influence of different cellular RNAs to the PPIase activity of hCyP33 was investigated using a protease-coupled method. The results show that hCyP33 binds specifically to mRNA, namely poly(A)(+)RNA, and that binding stimulates the PPIase activity of hCyP33.