The intercalation of thalidomide (alpha-phthalimidoglutarimide) into nucleic acids was studied by spectrophotometric titrations, displacement of known intercalators, viscosity changes of DNA solutions, and by phase partition analysis. Specific binding of thalidomide was found to DNA of various specimens (human placenta, calf thymus, salmon sperm), but not to RNA (from bakers yeast, torula yeast).