For use in human chemotherapy, there are several different substances for nearly each substance group available which cannot all be checked in routine susceptibility testing. If the bacterial resistance mechanisms and cross-resistance conferred by them are known, particular test substances can be selected and the results are interpreted on the basis of cross-resistance. Test substances correspond to those mentioned in guidelines for section sign 23 IfSG (German law on protection against infection). Due to suboptimal in vitro expression of different resistance mechanisms, it is necessary to perform additional tests besides routine agar-diffusion or microbroth MIC assays. These are preferentially tests for molecular demonstration of resistance genes. Emergence and spread of antibiotic-resistant nosocomial pathogens can be traced by typing. When selecting a typing method, it is important to assess work load, discriminatory power, and reproducibility. In future the availability of microarray technology will enable routine laboratories to demonstrate particular virulence-associated traits.