The 5'-termini of nucleolar 45S, 32S and 28S RNA's were analyzed by means of thin layer chromatography and Dowex-1 colum chromatography. 45S RNA did not bear a triphosphate at the 5'-terminus, but various monophosphates are found. 5'-termini of 32S and 28S RNA's were also heterogeneous. These results indicate that 45S molecules as isolated with the conventional procedure may not contains the primary transcript of the ribosomal gene, but a collection of large precursors with different degrees of processing at the 5'-terminus. The processing of the primary transcript may thus involve some unknown trimming processes at the 5'-terminus before the first major cleavage takes place.