A high heritability estimate was obtained for serum arylesterase activity level in a series of 40 male twin pairs (23 MZ, 17 DZ) aged 33–39 years. The heritability estimated for Cholinesterase activity was strikingly lower. In unrelated individuals a positive correlation between arylesterase, Cholinesterase, and various lipoprotein parameters appeared to support the suggestion that these enzymes interact with various lipoproteins at the molecular level. The nature of these associations is not known. It is, however, tempting to speculate that some of the hereditary influence on lipoprotein levels in man may be mediated through the genetic control of one or both of these enzymes.