Actinophage phi C31 was shown to be insensitive to a number of restriction and modification systems (RM). Phage sensitivity to RM systems of those strains to which phage cannot absorb, may be tested using protoplast transfection. For instance, the absence of phi C13 transfection in Streptomyces griseus Kr15 protoplasts, as compared to efficient transfection in protoplasts of R- M+ mutant of this strain seems to imply the sensitivity of phi C31 to the RM system of S. griseus KR15. Restriction of mutant phi C31 phage modified by S. albus G Rm system has been detected in S. coelicolor A3(2). This effect being dependent on a previous host may indicate that the mutant phage was rendered sensitive to an Rm system of S. coelicolor A3(2).