A method for targeting, treating, or diagnosing malignant mammalian tumor cells, comprising administering an effective amount of a .beta.1,6-branched oligosaccharide specific binding agent to the mammal. As a treatment, the binding agent may be intrinsically cytotoxic, initiate an endogenous cytotox ic cascade, or play a role in a cytotoxic cascade involving exogenous factors. A preferred binding agent is Bordetella pertussis, which is both specific for the .beta.1,6-branched oligosaccharide and well tolerated. Genetically engineered organisms may also be employed. Pharmaceutical compositions may also serve as binding agents.
