The plating efficiency of bacteriophage MX-1 on Myxococcus xanthus strains A and B and M. virescens V2 were compared. Comparison of strains V2 and A suggest that V2 is restrictive and A is not (restriction coefficient was approximately 8). A derivative of M. virescens V2 (strain V2-9) was obtained by repeated exposure of strain V2 to ultraviolet radiation. Strain V2-9 was also unrestrictive. Strain B is apparently unrestrictive too but analysis of phenotypic changes in phage derived from hosts V2, B and A suggested that some of the host-cell processes differ from orthodox restriction and modification. 2. Cell-free extracts from M. virescens V2 were fractionated by ion-exchange chromatography and two restriction endonucleases, R. MviV2I and R. MviV2II were identified. Nuclease I was found to hydrolyse coliphage lambdaDNA at apparently one site only and MX-1 DNA at approximatley 10 sites; nuclease II was found to hydrolyse MX-1 DNA at a very large number of sites and its restriction sequence was of comparable frequency with that of R. EcoRII. "Modified MX-1 DNA", obtained from phage whose last host was M. virescens V2 was hydrolysed by nuclease II but not by nuclease I. The significance of these findings for restriction in myxococci is discussed.
