SAKAMOTO, S. and OKAMOTO, R. Thymidine Kinase Activity in Familial Adenomatous Polyposis. Tohoku J. Exp. Med., 1992, 168 (2), 291-301-Thymidine kinase (TK) activity of polyp tissue from patients with familial adenomatous polyposis (FAP) was measured and compared with that of normal colon, sporadic polyp and colorectal carcinoma tissues. Total TK activity in colonic carcinoma was 3-fold that of normal; this increase seems attributable mainly to increased activity of cytosolic TK isozyme activity; the colorectal TK isozymes were separated into two types, i.e., fetal type and adult type isozymes predominantly in, respectively, cytosolic and mitochondrial fractions, by DEAE- cellulose column chromatography. However, FAP polyp samples from 15 patients showed an average elevation of only 1.8-fold over normal. Examined individually, only 5 of the 15 FAP samples showed significant elevations in total TK activity. Furthermore, TK isozyme analysis revealed variable patterns of the cytosolic isozyme activity being elevated in some cases (8 of 15) and remaining low in others. Thus FAP polyps seem to be a heterogenous population with respect to DNA replicative activity, and cytosolic TK isozyme activity may constitute a biochemical marker for the subsequent development of colorectal carcinoma in FAP.-familial polyposis coli (FPC); tymidine kinase (TK); isozyme