The serum of athymic nude mice bearing rat tumour xenografts has been examined for tumour-specific antigen. With a sarcoma and a hepatoma, tumour-specific antigen expression continued in xenograft growths, and sera of tumour-bearing mice contained free antigen, assayed by its ability to neutralise reactivity of tumour-immune rat sera against tumour target cells in an indirect membrane-immunofluorescence test. In contrast, no anti-rat antibody was detectable in sera of mice bearing the xenografts, or rejecting cells injected in admixture with BCG
