A general protein folding activity has been detected in the large subunit of the ribosome. This activity seems to be located in the peptidyl transferase domain of the large RNA of this subunit. In contrast to the protein folding activity of the molecular chaperones, this activity is (a) present in the RNA and is (b) universal, not selective for any protein. The overlap of this active site with the peptidyl transferase centre on the ribosomal RNA suggests a functional overlap between protein synthesis and folding by ribosome in the cell.