Intraspecific protoplast fusion was used to produce stable prototrophic recombinants of Streptomyces melanosporofaciens EF-76, a biocontrol agent of plant disease producing geldanamycin. Two recombinant strains (FP-54 and FP-60) that differed with regard to their antagonistic properties against Bacillus cereus ATCC 14579, Streptomyces scabies EF-35 and Phytophthora, fragariae var. rubi 390 were characterized. FP-60 lost the ability to inhibit the in vitro growth of these microbial strains while FP-54 exhibited higher antagonistic activities against them. FP-60 was deficient in geldanamycin biosynthesis whereas FP-54 was shown to produce, in addition to geldanamycin, at least two other antimicrobial compounds that were absent in the culture supernatants of strain EF-76. Like the wild-type strain EF-76, strain FP-54 reduced common scab symptoms on potato tuber but no significant difference was observed between the disease index attributed to tubers treated with strain EF-76 or with strain FP-54. Strain FP-60 showed no protective effect against common scab. The disease index of tubers treated with this recombinant was worse than the index associated with potato tubers from control treatments.