The present invention provides a simple and rapid method for preparing purified transposase complexes that are highly suited for fragmenting DNA. The method includes forming transposase complexes with oligonucleotide adapters in cell lysate, then purifying the complexes from the other substance in the cell lysate. Purification is accomplished using a specific binding pair, in which one member of the pair is bound to an oligonucleotide adapter of the complex and the other member of the pair is bound to a solid substrate. The bound complexes can be immediately used in DNA fragmentation reactions to produce solid substrate-bound DNA fragments, which can be used for any number of purposes, including as templates for amplification and sequencing.