3'-(2')-Amino- or thiol-modified, fluorescent dye-coupled nucleosides, nucleotides and oligonucleotides, and a process for the preparation and the use thereof.The OH group located in the 3' and/or 2' position of a nucleoside, nucleotide or oligonucleotide is derivatized to an amino or thiol group and subsequently a fluorescent dye is coupled thereto. The resulting 3'- and/or 2'-amino- and thiol-modified nucleosides, nucleotides and oligonucleotides can then be used for the synthesis of complementary strands in the presence of a template strand or of oligonucleotides and for the detection of genetic material. They have the advantage that the fluorescent label need no longer be attached to the 5' end of the oligonucleotide or to the nucleobase, and thus need not be introduced during the chemical synthesis as in labeling techniques hitherto known, while the known and conventional methods have the disadvantage that only a few polymerases can be employed for the synthesis, the acceptance of the triphosphates by the polymerases diminishes and, moreover, a large substrate excess is necessary.