Human melanoma, G361, which induces cachexia in nude mice, has been shown to produce a proteolysis-inducing factor (PIF) of Mr 24000, which is immunologically identical to that isolated from a cachexia-inducing murine tumour (MAC16). Biosynthetic labelling of G361 cells using a combination of [35S]sulphate and [6-3H]glucosamine gave a single component of Mr 24000 after affinity chromatography employing a murine monoclonal antibody. The material contained both radiolabels and, after digestion with peptide N-glycosidase F, two fragments were produced of Mr 14000 and 10000 also containing both radiolabels. Digestion with O-glycosidase produced three fragments of Mr 14000, 6000 and 4000, the first two of which contained both radiolabels, while the third only contained 3H. This digestion pattern is the same as that previously observed with PIF from the MAC16 tumour and is commensurate with one N-linked sulphated oligosaccharide chain of Mr 10000, one O-linked sulphated oligosaccharide chain of Mr 6000 and a central polypeptide chain of Mr 4000 with some residual carbohydrate. When PIF from G361 cells was administered to female NMRI mice (20 g) a pronounced depression of body weight (1.36+/-0.36 g; P < 0.0001 from control) was observed over a 24 h period without a decrease in either food or water consumption. Body composition analysis showed a significant decrease in the non-fat carcass mass without a change in carcass fat or body water. This result suggests that depletion of lean body mass in mice bearing G361 melanoma arises from the production of PIF.