Conditional mutagenesis in mice has recently been made possible through the combination of gene targeting techniques and site-directed mutagenesis, using the bacteriophage P1-derived Cre/loxP recombination system. The versatility of this approach depends on the availability of mouse mutants in which the recombinase Cre is expressed in the appropriate cell lineages or tissues. Here we report the generation of mice that express Cre in myeloid cells due to targeted insertion of the cre cDNA into their endogenous M lysozyme locus. In double mutant mice harboring both the LysMcre allele and one of two different loxP-flanked target genes tested, a deletion efficiency of 83-98% was determined in mature macrophages and near 100% in granulocytes. Partial deletion (16%) could be detected in CD11c+ splenic dendritic cells which are closely related to the monocyte/macrophage lineage. In contrast, no significant deletion was observed in tail DNA or purified T and B cells. Taken together, LysMcre mice allow for both specific and highly efficient Cre-mediated deletion of loxP-flanked target genes in myeloid cells.