Abstract
Hybridization of labeled HeLa cell messenger RNA to HeLa cell nuclear RNA occurs as determined by ribonuclease resistance of annealed mixtures of the two RNA's. Hybrids were characterized by melting temperature and by centrifugation to equilibrium in Cs
2SO
4 gradients. No annealing to nucleolar RNA, f2 phage RNA or homopolymers, was detected. The apparent complementarity of nuclear RNA and messenger RNA probably reflects an aspect of messenger RNA synthesis.