2-Aminopurine-modified abasic-site-containing duplex [DNA 5'-TCTGC GTCCT (PX) under barT TAACG CACAC-3'/3'-AGACG CAGGA T (C) under barA ATTGC GTGTG-5'; (P) under bar = 2-aminopurine, (X) under bar = abasic site (Spacer-C3), (C) under bar = receptor base] is capable of selectively binding to the bronchodilator theophylline with a dissociation constant of 10 mu M (5 degrees C, pH 7.0, I = 0.11 M) and is applicable to monitoring serum theophylline concentrations.