Streptomyces lividans 66 was shown to harbour two self-transmissible plasmids: SLP2, which acts as a sex factor, and SLP3. Derivatives of this strain which had lost both plasmids were used as host strains to study a range of Streptomyces plasmids for their ability to promote their own transfer and to mobilize chromosomal markers. A linkage map of the S. lividans chromosome containing ten markers was derived from the results of matings using several different sex plasmids, and protoplast fusions. SLP2 was transferred interspecifically to S. parvulus ATCC 12434 and to S. coelicolor A3(2); in the latter it acted as a fertility factor. Interspecific crosses also led to the discovery of a further plasmid, SLP4, from S. coelicolor, SLP2, SLP3 and SLP4 could not be visualized on agarose gels using standard plasmid isolation procedures, but their presence was detected by transformation into S. lividans.