Abstract: Regression associated antigens (RAAs) are identified in material from neoplastic cells by their immunological reactivity with regression associated antibodies from the serum of patients diagnosed as undergoing regression of a tumor. Regression associated antibodies (RAAbs) are identified by their absence during progression of a neoplastic disease state and by their presence in a diagnosed state of regression. RAAs have been purified and used to monitor the condition of cancer patients. Production of RAAbs and treatments employing those antibodies are described. It is herein disclosed that RAAs are expressed by M. hyorhinis and are also expressed by expression of provided nucleotide sequences in recombinant host cells, particularly nucleotide sequence for 38 kd and 43 kd RAAs as expressed in E. coli. RAAs and nucleic acids encoding RAAs (or portions thereof) and RAAbs may be used in diagnostic assays and immunotherapy.