Seventy nine strains of salmonella of human origin were tested for antibiotic-resistance. Eighteen strains (11 Salmonella typhi-murium, 4 S. saint-paul and 3 S. dublin) were multiresistant, particularly to chloramphenicol. All the strains were able to transfer their resistance by conjugation. Thirteen conjugation R plasmids were classified into five incompatibility groups: I1, N, H1, M and B. Molecular studies showed that the DNA structures of the four IncI1R plasmids differed from one another and also partly from the plasmid DNA structures of other IncI1R plasmid DNA isolated from salmonellae. In conclusion, chloramphenicol resistance was conferred by various R factors, and the R factors of the I1 incompatibility group appeared to have different plasmid structures.