pSLH8, an insertional-inactivation cloning vector for Escherichia coli has been constructed by inserting a pigment gene (probably encoding an indole dioxygenase) from Rhodococcus sp. ATCC 21145 into pUC18. Wild-type E. coli colonies containing pSLH8 produce insoluble indigo and turn dark blue on unsupplemented LB agar. Insertion of DNA fragments into the unique BamHI, EcoRI, EcoRV, HindIII, PstI, SphI and SstI polylinker cloning sites disrupts the reading frame of the fully sequenced pigment gene and results in unpigmented colonies. pSLH8 may be an attractive alternative to pUC18 and similar plasmids because it does not require specifically mutated host strains or an expensive substrate for colour development.