A human melanoma cell line, SEKI, induces severe cachexia in tumor-bearing nude mice. A factor with the ability to inhibit lipoprotein lipase (LPL) was isolated from the conditioned medium of this cell line. This factor was 40-K-dalton protein, and designated temporarily as melanoma-derived LPL inhibitor (MLPLI). Amino acid sequencing revealed that the amino-terminal portion consists of SPLPITPV-AT--IR-P. Unexpectedly, the sequence, as far as determined, was identical to those of leukemia inhibitory factor (LIF), suggesting that MLPLI is a protein closely related to LIF. The findings that MLPLI inhibits LPL activity and that MLPLI is produced by human cancer cells inducing cancer cachexia also suggest that this protein is a candidate for the factor responsible for cancer cachexia.