Double stranded DNA from calf thymus was irradiated in the presence of bovine serum albumin with a ratio of 1:10 in weight under aerobic and anaerobic conditions. The irradiated biomolecules were separated by gel filtration on Sepharose Cl-2B with and without sodium dodecylsulphate. By scavenging OH-radicals the protein protects DNA, but in contrast to radiolysis of DNA in phosphate buffer, in the presence of serum albumin oxygen enhances the degradation of DNA. Radiolysis under N2 leads to crosslinking of serum albumin to DNA with higher yields at pH 5 than at pH 7. Oxygen largely prevents crosslinking.