A procedure for the nonradioactive labeling of oligonucleotides with the hapten digoxigenin (DIG) has been developed. The label is introduced by enzymatic tailing of the 3'-end. Two different modified nucleotides were applied. DIG-dUTP allows the synthesis of hapten-modified oligonucleotides with longer tails containing several DIG molecules, whereas the novel compound DIG-ddUTP leads to the addition of only a single DIG hapten. The efficiency of the labeling reactions with respect to variation of the different parameters was analyzed and data on application of labeled oligonucleotide probes to different blot formats are shown.