Transgenic animal mutagenesis assays using lambda shuttle vectors have recently been described for isolation and characterization of spontaneous and chemical induced DNA mutations. Extensive information on lambda and E. coli genetics provides a wealth of techniques to allow selection of mutant target genes. Here we describe the modification of an E. coli host which permits two methods for the direct selection of mutant genes. These methods reduce the number of plates needed to be screened for a comparable amount of frequency data by 20-100-fold and thus provide a significant savings of the materials and time required for the screening of mutations. In addition, mutants selected by these approaches described here may alter or broaden the spectrum of mutations that are recoverable. Ultimately, a combination of selective and nonselective techniques may prove valuable for the analysis of mutations produced in vivo in transgenic animals.