Existence of lipoprotein lipase in human sarcomas and carcinomas. Aqueous extracts of acetone/ether powders of surgically obtained specimens of human tumors hydrolyzed 3H-labeled triolein in a dose-dependent manner. The lipolytic activity in these extracts was inhibited by anti-lipoprotein lipase (LPL) IgG dose-dependently, 25 micrograms of anti-LPL IgG causing 95% inhibition of the activity. Thus, LPL accounts for most of the lipolytic activity in extracts of acetone/ether powders of the tumors. All sarcomas and carcinomas examined contained LPL activity. Western blotting showed that they gave a band corresponding to that of human adipose tissue LPL (M(r) = 57,000). Immunocytochemical studies showed that LPL was present in cultured human osteosarcoma cells and distributed throughout the cells. We determined the proliferating cell nuclear antigen (PCNA)-labeling index as an indicator of the proliferative activity of tumor cells and measured LPL activity in extracts of tumors in areas corresponding to those used for determining the PCNA-labeling index. In malignant fibrous histiocytomas, the PCNA-labeling index in area a, which corresponds to the subcapsular region, was higher than that in area b, which corresponds to the central region. The LPL activity in area a was 10 times that in area b. In rectal cancer, the index in area c, which corresponds to the subserosal region, was higher than that in area d, which corresponds to the submucosal region. The LPL activity in area c was 1.9 times that in area d. These findings indicate heterogeneity in the distributions of LPL activity within tumors and higher levels of LPL activity in tumors that are proliferating actively.