A subline of the human B cell lymphoma DHL-4, grown in the artificial serum-free medium HB101, displayed a resistant phenotype to the activity of an antisense oligodeoxynucleotide (aODN) effective on the parental DHL-4 line. It was found that the cellular uptake of the 18mer aODN in the two cell lines was almost the same. In contrast, the unresponsive subline DHL-4r degraded the aODN very efficiently, in contrast to the stability of aODN inside cells of the parental DHL-4 line. Activation of the degrading 'machinery' combined with selective properties of the artificial medium may be responsible for the loss of responsiveness to aODN.