The effect of salt concentration, ion type, pH, and solvent upon poly U melting temperature, Tm, has been investigated. Alcohol increases the Tm of poly U. The Tm depends on the type of ion present and salt concentration in a way which is different from that observed for the RNA of R17 virus. Ultracentrifuge data and the dependence of the Tm upon KCl concentration suggest that the melting of poly U is a complex phenomenon.