Action spectra for the effect given by u. v. radiation on the DNA molecule from phage $\alpha$ irradiated in vitro. - Action spectra have been obtained concerning the effect of ultraviolet radiation on the physico-chemical properties of the DNA of phage $\alpha$ irradiated in vitro. Through the use of the analytical ultracentrifuge and of the spectrophotometer the following effects have been studied: a) production of interruptions in the single polinucleotide strand; b) local denaturation; c) presence of a fraction of molecules resistent to denaturation; d) increase in the bouyant density of irradiated DNA. All of the action spectra thus obtained show a slight variation of the radiation efficiency in the range between 2600 and 2800 Å, and a well defined peak at $\lambda \simeq 2880$ Å. Threshold values for all of the examined effects are at 3000 Å.