R factors from Proteus mirabilis and P. vulgaris.

Eighty-nine R factors were transmitted by conjugation to Escherichia coli K12 from isolates of Proteus hauseri (P. mirabilis plus P vulgaris). More than half were non-selftranmissible. The remainder included plasmids assigned to the previously defined groups FII,A-C complex, J, N and P, as well as some not belonging to any knwon compatibility groups. R factors from strains isolated in India, Thailand and Japan carried plasmids whose inheritance was extremely unstable in E. coli K12. All belonged to a new compatibility group, V.