Cross-resistance is the tolerance to a usually toxic substance as a result of exposure to a similarly acting substance. It is a phenomenon affecting e.g. pesticides and antibiotics. As an example rifabutin and rifampin cross react in the treatment of tuberculosis. This sort of resistance is also seen against drugs involved with the treatment of HIV. Drugs ending with "- ine" and "-inavir" are the most common.
Sometimes also seen in Antibiotics in animal feed and spread of resistant campylobacter from poultry to humans, where genetic code of organism jumps from Ciproflxm resistant to other, therefore cross resisting.