We have explored the potential of the Tn 552 in vitro transposition reaction as a genetic tool. The reaction is simple (requiring a single protein component), robust and efficient, readily producing insertions into several percent of target DNA. Most importantly, Tn 552 insertions in vitro appear to be essentially random. Extensive analyses indicate that the transposon exhibits no significant regional or sequence specificity for target DNA and leaves no discernible 'cold' spots devoid of insertions. The utility of the in vitro reaction for DNA sequencing was demonstrated with a cosmid containing the Mycobacterium smegmatis recBCD gene cluster. The nucleotide sequence of the entire operon was determined using 71 independent Tn 552 insertions, which generated over 13.5 kb of unique sequence and simultaneously provided a comprehensive collection of insertion mutants. The relatively short ends of Tn 552 make construction of novel transposons a simple process and we describe several useful derivatives. The data presented suggest that Tn 552 transposition is a valuable addition to the arsenal of tools available for molecular biology and genomics.