T4 polynucleotide kinase has been used to end-label specific RNA purified by multiple hybridizations to nitrocellulose-bound DNA. The pico moles of ends of a specific mRNA transcribed from the chromosome, even from several liters of Escherichia coli, give concentrations perhaps 2000-fold below the Km value of the kinase-RNA substrate. In such a reaction, optimal incorporation was observed with increasing ATP concentration to greater than or equal to 7 microM (greater than or equal to 15 mCi of carrier-free [32P]ATP in a 300-500 microliter reaction). The unreacted ATP (greater than 150-fold excess) could best be eliminated by multiple gel filtrations rather than by precipitation, ion exchange chromatography or dialysis. The [5'-32P]RNA was digested with T1 or pancreatic RNase and the [5'-32P]oligonucleotides separated by size in a 20% polyacrylamide gel. Oligonucleotides of a specific size were separated sufficiently by a second dimension electrophoresis on cellulose acetate. We have used partial alkali digestion in sequencing the purified oligonucleotides. As opposed to other digestions, alkali produces 5',3'-diphospho-oligonucleotides whose mobilities can differ from those of the monophosphates, e.g., much longer running times in conventional homochromatography.