Abstract: A method of determining the immune response of a mammal to circulating tumour marker proteins is described in which a sample of bodily fluid, for example plasma or serum, is contacted with a panel of two or more distinct tumour marker antigens. The presence of complexes between the tumour marker antigens and any autoantibodies to the antigens present in the sample are detected and provide an indication of an immune response to a circulating tumour marker protein. The method is useful for the diagnosis of cancer, particularly for identifying new or recurrent cancer in an otherwise assymptomatic patient.