A number of non auto-transferring plasmids (NTP) coding for drug resistance or for colicin synthesis were examined in Escherichia coli and salmonellae. These plasmids form Class 2 transfer systems in which the resistance or colicin determinant(s) and the transfer factor are discrete plasmids, independent of each other in the host cell. The transfer frequency of the determinant depends on the type of transfer factor used for its mobilisation. For example, the streptomycin-sulphonamide resistance (SSu) determinant was transferred more efficiently by I-like transfer factors than by F-like plasmids. The plasmids NTP1 to NTP11, of which eight coded for drug resistance and three for colicinogeny, fell into at least six compatibility groups. A number of wild SSu-resistant strains of salmonellae and E.coli carried non-transferring determinants incompatible with the ampicillin-sulphonamide resistance (ASu) determinant. These SSu plasmids are probably phylogenetically related to the prototype SSu determinant from which ASu was derived. NTP1 to NTP11 consist of covalently closed circular DNA molecules with mean contour lengths between 2.22 and 4.53 mm. All are present in multiple copies per chromosome in E.coli K12. A non-transferring kanamycin resistance determinant K reversed the "fertility inhibition +" (fi +) property of Salmonella typhimurium phage type 36, to make it fi -. K had a similar effect on at least fourteen other strains of S. typhimurium. Examination of five fi+ S. typhimurium strains and one fi- strain demonstrated that the fi+ property was dependent on the presence of a plasmid, which was designated MP10. Molecular studies confirmed that K was incompatible with MP10 and the two plasmids showed a high degree of DNA homology. K was probably formed by recombination between a kanamycin resistance determinant and the MP10 plasmid of the original S. typhimurium host. The K plasmid is present as approximately one copy per chromosome and it has a mean contour length of 18.1 mm. These results establish that there are at least two sorts of non auto-transferring plasmids. Plasmids of the first type are small, less than 5 pm in length, and exist in multiple copies per chromosome. In contrast, members of the second type are usually much larger in size and are present as approximately one copy per chromosome. This thesis is a report of research performed by the author in the Enteric Reference Laboratory, Central Public Health Laboratory, Colindale, and in the School of Pharmacy, University of London from 1970 to 1975. Accounts of parts of this work have been published as follows: Anderson, E. S. and Smith, H. R. (1972). Fertility inhibition in strains of Salmonella typhimurium. Molec. Gen. Genet., 118, 79-84. Smith, H. R., Grindley, J. N., Grindley, N. D. F. and Anderson, E. S. (1970). Depression of F-lac in Salmonella typhimurium by a determinant for kanamycin resistance. Genet. Res. Camb., 16, 349-353. Smith, H. R., Humphreys, G. 0., Grindley, N. D. F., Grindley, J. N. and Anderson, E. S. (1973). Molecular studies of an fi+ plasmid from strains of Salmonella typhimurium. Molec. Gen. G.net., 126, 143-151. Smith, H. R., Humphreys, G. 0. and Anderson, E. S. (1974). Genetic and molecular characterisation of some non-transferring plasmids. Molec. Gen. Genet., 129, 229-242.