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Introduction4-Chloroindole-3-acetic acid (4-Cl-IAA), isolated from immature seeds of Pisum sativum, [1][2][3] has strong hormonal activity as compared to IAA. [4][5][6][7][8][9][10][11] Its synthesis and biological activities and those of its esters also have been reported. 12,13) Root formation-promoting activities of 4-Cl-IAA and its esters are much higher than the activity of 4-(3-indole)butyric acid (IBA), the active ingredient of a commercially available root formation-promoting agent, the respective promoting activities of 4-Cl-IAA and its ethyl ester being 3.3 and 3.5 times that of IBA for Serissa japonica. 12) We are interested in massproducing tree saplings for reforestation by means of compounds that have strong root formation-promoting activity. We reported that 4-Cl-IAA, 5,6-dichoroindole-3-acetic acid (5,6-Cl 2 -IAA) and their ester derivatives are promising candidates for root formation promoters.14-16) They strongly promote the root formation of tree cuttings by both the soaking and spraying methods. In particular, root formation by spraying has an important advantage because it is easy to massproduce tree saplings for reforestation by that method.16) In root formation by Serissa plants, 4-Cl-IAA esters that bear one to three carbons in the alcohol part of the molecule also have strong root formation-promoting and other plant growthregulating activities.12,13) Our interest is in more polar derivatives bearing three carbons among esters with one to three carbons in the alcohol part of the 4-Cl-IAA ester. We selected lactic acid derivatives of 4-Cl-IAA, polar three-carbon derivatives that bond directly to 4-Cl-IAA, as new candidates for plant growth regulators. The synthesis and plant growth-regulating activities of these L-lactic acid derivatives (1a-1k) of 4-Cl-IAA are reported. The ester derivatives of 4-chloroindole-3-acetic acid (4-Cl-IAA) with L-lactic acid and L-lactic acid esters were synthesized by two schemes from 4-Cl-IAA and L-lactic acid or its esters. Their biological activities were determined by three bioassays (hypocotyl growth inhibition in Chinese cabbage (Brassica pekinensis cv. Kinshu), hypocotyl swelling and lateral root formation in black gram (Vigna mungo (L.) Hepper), and adventitious root formation in black gram cuttings). All the L-lactic acid ester derivatives had strong activities in the three bioassays, but their activities were somewhat weaker than those of 4-Cl-IAA in the first two bioassays. Root formation activity of some esters in cuttings was higher than, or the same as, that of 4-Cl-IAA. At 1ϫ10 Ϫ5 M, roots induced by the esters were about 2 to 4 times those of 4-(3-indole)butyric acid (IBA). Unexpectedly, the free lactic acid derivative had the weakest activity of all the lactic acid derivatives tested in all bioassays. Synthesis and plant growth-regulating activities of L-lactic acid derivatives of 4-chloroindole-3-acetic acid