The inhibitory activities of a set of nine 7-substituted-4-hydroxyquinoline-3-carboxylic acids against three dehydrogenase enzymes and one whole cell system (Ehrlich ascites tumor cells) have been subjected to principal component analysis. The results clearly indicate that activity against the whole cell test system cannot directly be attributed to inhibition of the enzymes evaluated. The enzyme systems are reflected by the first component that can be identified with polar and steric parameters while hydrophobic effects are absent. The second component is entirely due to the inhibition of ascites cell respiration that depends primarily on hydrophobicity.