The structural relationships of 30 straight-chain fatty acids and derivatives and their bactericidal properties were studied with 8 gram-negative and 12 grampositive organisms. Chain length, unsaturation (cis, trans), and functional group were all variables considered in this study. Our data indicate that C(12) (lauric acid) is the most inhibitory saturated fatty acid against gram-positive organisms. Monoenoic acid (C(18:1)) was more inhibitory than saturated fatty acid, but was less active than dienoic derivatives (C(18:2)). Other unsaturated compounds were less active than C(18:2). Alcohols and glyceryl esters were active only against gram-positive organisms. In general, esterification of the carboxyl group led to a compound which was less active; monoglycerides were the sole exception. Amine derivatives, contrary to results with fatty acids, esters, and amides, showed activity against both gram-positive and gram-negative organisms.