Abstract: Diagnostic methods comprise measuring specifically the level of at least one iso-eosinophilic cationic protein (iso-ECP) in a sample from an individual to be diagnosed, and comparing the measured level of the iso-ECP with a predetermined level of the iso-ECP. The iso-ECP is cytotoxic and the cytotoxicity of the iso-ECP is not capable of neutralization by the monoclonal antibodies EG1 and EG2. Anti-iso-ECP antibody which may be used in the diagnostic methods specifically binds to a cytotoxic isoform of ECP having an epitope which is unique for the native form of the cytotoxic iso-ECP.