Abstract: A method for reducing background caused by target-independent generation of amplification products, typically the products of a ligase chain reaction or a polymerase chain reaction, involves chemically "masking" or blocking the amplification probes or primers so that they cannot be extended or ligated until the occurrence of a triggering event which can be delayed until the amplification reaction is begun. The probe masks take the form of complementary blocking oligonucleotides that are incapable of serving as template themselves and inhibit random tailing of the probe/primers. The blocking oligo masks are denatured from the probes during amplification and preferably are effectively eliminated from competing for probes in the amplification reaction.