Ten of the 33 proteins found in the 50 S subunit of the Escherichia coli ribosome associate independently and specifically to the 23 S ribosomal RNA [l]. The location of the binding sites for these proteins has been studied in two different ways. Branlant et al. [2,3] have isolated ribonucleoprotein particles resulting from extensive nuclease digestion of complexes between proteins Ll, L20, L23, L24 and the 23 S RNA. Sequence analysis of the RNA regions protected from digestion by the proteins enabled these authors to locate the binding sites on the 23 S RNA molecule. On the other hand, Spierer et al. [4] have tested the association of each of the ten binding proteins with two segments of the 23 S RNA produced by limited ribonuclease digestion of the 50 S subunit. One segment sediments at 13 S, covers 40% of the 23 S RNA sequence, and derives from the 5' end of the molecule. It contains the specific binding sites for proteins LA, L20 and L24. The other segment, which sediments at 18 S, encompasses about 60% of the 23 S RNA sequence and arises from the 3' end of the molecule. It contains the specific binding sites for proteins Ll, L2, L3, L6, L13, L16 and L23. We report here the preparation of fragments of the 18 S RNA segment, and the distribution of binding sites for proteins Ll, L.3, L6, L13 and L23 on these fragments.