Abstract2,2′‐Di(carboxymethylthio) pentan ‐4‐one was found to increase the stem height of intact tomato plants growing in pots under glass. Stem height was increased by 60 percent although this was accompanied by malformations in lamina development, leaf epinasty, and a final reduction in shoot weight. Stem growth of tobacco and potato were also stimulated by this compound but the deleterious effects on leaf development were less marked. Subsequent screening of chemical analogues, using as an assay the growth response of intact tomato plants revealed that 3,3′‐di(carboxymethylthio)heptan‐5‐one initially stimulated stem growth but then produced severe shoot damage, similar to that caused by phenoxyacetic acid herbicides. Both chemicals stimulated extension growth in a wheat coleoptile section test though to a lesser extent than indole‐3‐acetic acid.Like the carboxymethyl‐N:N‐dialkyldithiocarbamates, these straight chain compounds appear to possess true auxin activity that could be related to the formation of an incomplete un