Tautomer-specific monoclonal antibodies (mab) to N-3-substituted adenines were generated, which bound the modified adenines with a high affinity regardless of the size of the groups on position 3 of the adenine. N-3-substituted haptens were prepd. and bound to a carrier protein, then used to induce an immune response. Mabs with affinity consts. for 3-ethyladenine in the region of 1010 L/mol were generated. The antibodies demonstrated a high specificity for N-3-substituted adenine derivs. These mabs might be useful in the purifn. of N-3-substituted adenines from body fluids or in the monitoring of chemotherapy with alkylating cytostatic agents.