The development of antimicrobial drug resistance during 1974 to 1995 of the three most common Salmonella serovars S. enteritidis, S. typhimurium and S. typhimurium O:5 negative is described. In general the resistance rate for Salmonella is around 20%, peaking with 40% in 1990 when multiresistant S. typhimurium O:5 negative DT 204 prevailed. In S. enteritidis the resistance rate is comparably low (< 10%). In order to gain more information on resistance in S. enteritidis, the detailed phenotypic and genetic properties of 65 selected isolates were investigated by genetic and molecular techniques. It could be shown, that S. enteritidis isolates expressed resistance to antibiotics which are commonly used in veterinary medicine, namely tetracycline, ampicillin, chloramphenicol, kanamycin and gentamicin. The resistance phenotypes turned out to be very stable and from the 65 investigated strains 50% were monoresistant, and 27.7% of them exhibited resistance against tetracycline only. Another 50% of the strains were multiresistant to 2 or more antimicrobial agents. 61% of them were resistant against tetracycline and kanamycin. Mating experiments were carried out, in order to detect the presence of R-factors possessing transfer systems at 37°C and 22°C, reflecting conditions in two habitats that Salmonella serovars accounter during their life-cycle. High temperatures in the host and low in the environment. 83% (54) of all investigated strains transferred one or more resistances under the mating conditions applied. The majority of 71% (46) transferred at 37°C and 12% (8) of all strains transferred their resistances at 22°C preferentially. In order to determine a clonal structure among the strains, further strain- differentiation was achieved by plasmid-profile-analysis and phagetyping. All isolates carried one or more plasmids and the whole population showed a very heterogeneous plasmid distribution. 69% carried the virulence plasmid pRQ29 and a 70-75 MD plasmid was present in 54% of the strains. In conclusion the plasmid-profiling combined with the results of the antibiotic resistance tests and phagetyping elucidated a very heterogeneous population structure. The presence of clones was limited to only 6 strains. This contrasts the results obtained with other multiresistant serotypes e.g. the dominating multiresistant clone S. typhimurium DT104. However, multiresistant S. enteritidis strains exist in Germany and selective pressure could facilitate their spread. This imposes the danger, that a mainly antibiotic sensitive S. enteritidis population might be substituted with resistant strains. Since 1991 there has been an increase in the rate of Nitrofurantoin-resistance in S. enteritidis from 0% in 1990 to 35% in 1995. 50% of the S. enteritidis strains originated from poultry, and they showed a resistance level of 72% against Nitrofurantoin. These strains appear to be epidemiologically related and might be derived from a clonal spread of a single ancestor. Nevertheless further molecular methods should be applied in order to determine the source of these nitrofurantoin resistant strains.