Genetically engineered E. coli amended into a non-sterile soil microcosm were able to mobilize their recombinant plasmid DNA into a plasmidless E. coli recipient. This transfer required participation of a mobilizer bacterium containing a self-transmissible (conjugative) plasmid. Mobilization also depended on mob sequences present on the non-conjugative recombinant plasmid. Sequences from the non-conjugative plasmid pHSV106, which contains the herpes simplex virus thymidine kinase gene, were identified by DNA hybridization in recipient cells isolated from the soil.