A sensitive and simple method for the quantitation of oligonucleotide phosphorothioates in biological fluids and tissues is described. This method is based on the extraction of the oligonucleotide from the biological fluids and tissues and immobilization on a nylon membrane. The membrane-bound oligodeoxynucleotide phosphorothioate is then hybridized with labeled complementary oligonucleotide and exposed to X-ray film. The data on the film can be scanned and used to create a standard curve. The sensitivity of detection by the method described here will be useful to monitor the pharmacokinetics of oligonucleotides in bodily fluids and distribution in various tissues. The results indicate that the method is rapid and allows handling of a large number of samples at the same time.