A number of 3-carbonylacrylic acid derivatives were prepared, with a view to varying systematically the stereoelectronic environment of the conjugated double bond. The rates of reaction with cysteine were measured spectrophotometrically when possible or by stopped flow when very fast. Some of the final reaction products were isolated. Other properties examined were partition substituent constants and antimicrobial activity. On the basis of published data and these studies, the activity appears to be the combined effect of at least two mechanisms, one probably related to the effect of these structures on surface tension, the other to the electrophilic properties of the unsaturated system.