AbstractThe possibility that small nuclear RNA species U1 might be involved in the inhibition of protein synthesis that occurs during mitosis has been explored. Upon exposure of mitotic HeLa cell extracts to 1% sodium deoxycholate, the majority of the rapidly sedimenting U1 RNA shifted to lower sedimentation rates. This suggests that it is associated with heterogeneous nuclear RNA ribonucleoprotein particles, instead of a ribosomal population. Erythrocyte ghost‐mediated microinjection of anti‐(U1)RNP antibodies into synchronized HeLa cells did not prevent the suppression of protein synthesis that is observed under mitosis. Examination of the published nucleotide sequences of U1 and U2 RNA suggests that these RNA species could potentially code for some short peptides. When purified U1 or U2 RNA were added to cell‐free polypeptide synthesizing systems, the synthesis of these peptides was not det