A lipid mobilizing factor has been purified from a cachexia-inducing mouse colon adenocarcinoma (MAC16) using a combination of ion exchange (Mono Q), exclusion (Superose) and reverse phase hydrophobic chromatography. The purification process led to a 3,500-fold increase in the specific activity. Serum from mice bearing the MAC16 tumour contained antibodies reactive with fractions containing lipid mobilizing activity and detectable as a 24 kDa immunoreactive band on Western blotting. Serum from mice transplanted with a related tumour, MAC13, not producing cachexia, did not contain antibodies. A similar immunoreactive band was detectable in the urine of patients with cancer cachexia, but was absent from the urine of normal subjects. A monoclonal antibody produced by fusion of splenocytes from mice bearing the MAC16 tumour with mouse Balb/c myeloma cells attenuated the development of cachexia in mice transplanted with the MAC16 tumour and inhibited tumour growth. These results suggest that the M(r) 24 kDa antigen may be important in tumour growth and cachexia.