To construct a science of quantitative structure-activity relationships (QSAR) we must begin to do comparative studies of QSAR in many different systems. In this way we can begin to understand the similarities in different compounds interacting with the same biological system and different biological systems interacting with the same compounds. Examples are given for the inhibition of insect and vertebrate cholinesterase by a variety of chemicals. For the inhibition of vertebrate enzyme the hydrophobic properties of the inhibitors is important, but not for fly enzyme.Since the initiation of the QSAR paradigm in 1962 (7) thousands of quantitative structure-activity relationships have been published for all kinds of organic compounds acting on all sorts of organisms or parts thereof (DNA, enzymes, organelles, etc.). In addition to these, physical organic chemists have been hard at work since 1935 formulating quantitative relationships for organic reactions in solution. Our own data bank, which is only a fraction of the literature, contains about 6,000 examples about equally divided between biological and organic chemistry. The time has come to begin the serious organization and generalization of this mass of published and still rapidly growing work Already it is possible to make interesting comparisons and generalizations (2-5). We believe that it is particularly important to tie the less certain biological QSAR to the more firmly based relationships established by physical organic chemists (2, J). Before considering the central topic of this report, it helps to illustrate our general approach with a few diverse examples.Many years ago (3) we derived equation 1 for the colchicine-like mitosis in onion root tips caused by simple organic solvents such as: alcohols, acetone, CHCI3, xylene, ether, etc.log 1/C = 0.95 log Ρ + 0.63 η = 22, r* = 0.916, s = 0.381 (1)In this equation, C is the molar concentration of chemical producing the standard aberrant mitosis. This seemed a highly esoteric study only of academic interest, still we entered it into our bank. Recently, we came across equation 2 by Onfelt