Agrobacterium tumefaciens is best known for its ability to transform plants by delivering the T-DNA that is processed and transferred from the resident Ti plasmid to the recipient plant cells. Less well known is the capacity of this Gram-negative bacterium to transfer its T-DNA into fungi and actinomycetes. Procedures are described on the use of the promiscuous T-DNA transfer system of A. tumefaciens to transform members of the actinomycetes. Integration of the T-DNA derivatives into the Streptomyces chromosome provides opportunities of obtaining valuable mutants of this antibiotic producing soil organism.