Conditions for highly efficient genetic recombination in Streptomyces by protoplast fusion are described. Protoplasts of S. fradiae and S. griseofuscus were formed by a modification of the glycine-lysozyme-lytic enzyme method (Okanishi, Suzuki & Umezawa, 1974). Regeneration of cells from protoplasts was monitored throughout the growth cycle and was most efficient when cells of either S. fradiae or S. griseofuscus were taken from the transition phase between the exponential and stationary growth phases. Fusion of protoplasts carrying different auxotrophic or chromosomal drug-resistance markers was achieved by treatment with polyethylene glycol, and high frequencies of stable genetic recombinants were obtained. Strain development and genetic analysis of several other economically important Streptomyces species, however, has been hindered by the apparent lack of natural fertility in these strains, and the lack of transduction and transformation systems (Hopwood et al., 1973). A more general means to effect genetic recombination in Streptomyces would therefore be quite useful.Polyethylene-glycol-induced protoplast (or cell) fusion has recently been demonstrated in several eukaryotic systems (Ferenczy, Kevei & Szegedi, 1975; Ann6 & Peberdy, 1976;Jones et al., 1976;Power et al., 1976;Pontecorvo, Riddle & Hales, 1977;Sipiczki & Ferenczy, 1977) and in the prokaryotic genus, Bacillus (Fodor & Alfoldi, 1976;Schaeffer, Cami & Hotchkiss, 1976). This technique appears to have potential broad utility to effect genetic recombination in prokaryotic micro-organisms, particularly in those lacking classical recombination systems. A key factor in the utility of this technique is the capability of fused bacterial protoplasts to regenerate cell walls and thus viable cells. In this communication, I describe a highly efficient genetic recombination system for Streptomyces using protoplast fusion and efficient cell regeneration.Downloaded from www.microbiologyresearch.org by