A DNA fragment containing a 16 nucleotide sequence mimicking the intron extremities of premessenger RNA aligned as proposed previously (1,2) in a model of splicing mechanism was prepared and used as a probe for accessibility of the 5' extremity of U1 RNA. Hybridization of U1 RNA to the probe under non denaturing conditions and digestion of the hybrid with RNase H revealed that the sequence of U1 RNA which is complementary to the extremities of introns is accessible to hybridization and to enzymes. Therefore, the configuration of isolated U1 RNA satisfies the criteria required for the alignment of introns and further enzymatic reactions of splicing.